To answer this question, it is first important for one to understand that the general concept of a month originated with the lunar cycle -- the amount of time it takes for the moon to circle the earth -- and the general concept of a year originated with the solar cycle -- the amount of time it takes for the earth to circle the sun. In that it takes the moon approximately 29.5 days to complete its cycle and it takes the earth approximately 364.25 days to complete its cycle, a standard of 12 months in a year developed – but this is not exact. This is the issue with the Jewish calendar that leads to the need for a leap year.
Some calendars were established with the solar cycle as the base, with months determined by simply dividing the year by 12. This is the case with our conventional Western calendar of 12 months with, generally, more days in a month than are in the lunar month. The result is that our months are not connected in any way with the movement of the moon. A new moon may fall on any day of a month.
Other calendars use the lunar cycle as the base with the years simply consisting of 12 lunar months. This is the case with the Moslem calendar with a year consisting, as such, of fewer days than the solar year. The result is that this yearly cycle is not connected with the movement of the sun, resulting in no month specifically being tied to a certain season. While January of a conventional calendar will always be in the winter, a Moslem month may, at times, be in the summer and, at other times, be in the winter.
The challenge with the Jewish calendar is that while it defines its months clearly by the lunar cycle, the Torah’s further insistence that the holidays be in certain seasons – such as Passover in the spring – demand of us to also consider the solar year. Simply, the Hebrew month of Nissan must always be in the spring. The result is that 7 times in 19 years, an additional month – the leap month of Adar 1 – has to be added to the Jewish calendar to bring the holidays in line with the seasons. These leap years consist of 13 months and we are presently in one of these leap years.
The further question you asked, though, was: why was this year specifically a leap year? The fact is that originally, the determination of a leap year was made by the Sanhedrin (or a committee thereof) judicially as per the need. See Mishnah Sanhedrin 1:1; Rambam, Sefer HaMitzvot, Aseh 153 (with Ramban); and Sefer HaChinuch, Mitzvah 4. As those who made such decisions had a strong knowledge of astronomy, the idea that leap years had to occur 7 times in 19 years must have been known and would be considered a factor in making such decisions. It is generally understood, though, that in times when the Sanhedrin functioned the decision on a leap year was made judicially in response to need and in the year in which the extra month would be added.
We have a tradition, though, that towards the end of the 4th century, Rabbi Hillel HaSheni, the Nasi [President] of the Sanhedrin at that time, in response to a concern for persecution of the Jews, computed a calendar that would establish the future dates for observance of the holidays. This calculation included the formula of leap years 7 times in 19 years. Our present Jewish calendar is the product of this calculation and so it was effectively mandated, in this manner, that this year would have an extra month, i.e. be a leap year.
To be honest, however, this does not really fully answer the question. The further question may be: why did God create the lunar and solar cycle in this way thus necessitating such calculations? Often, when we ask questions, we assume the facts about reality to be a given and then ask for an explanation of the response. That is the simple way of addressing a question such as this one: Given the astronomical reality and the needs within the Jewish calendar to both incorporate the lunar month and the solar seasons, what was done to accommodate these requirements? The challenge is that we also believe that God is the One Who created these lunar and solar cycles – so we may then wonder: why did He not create them to be more aligned? Why did God create the world in such a manner that the calendar would demand such mathematical and judicial dexterities? This is a question that I now leave you with.
The Jewish calendar is a lunar/solar calendar. It is lunar in that each month is measured by the waxing and waning of the moon (approximately 29-30 days beginning with the appearance of the new moon). It is solar in that the length of the year is based on the earth’s orbit around the sun so that the holiday cycle is linked to the seasons.
A problem arise, however, because the length of twelve lunar months is about 11 days shorter than the solar year (one orbit of the earth around the sun takes about 365.25 days; that is why in the civil [Gregorian] calendar we add 1 day every 4 years to make up for the quarter day we lose every calendar year.) If uncorrected, the Hebrew calendar would get off track with the earth’s orbit by about 11 days. But the Torah also requires that we celebrate our holidays in the correct season. In addition to their historical and spiritual significance, each holiday is tied to the agricultural cycle. Passover must happen in the spring, Shavuot in the summer, and Sukkot in the fall. If we didn’t adjust the calendar, the holidays would drift out of sync with the seasons. To correct for that, we add an extra month every 2 or 3 years. The Jewish "leap year", which occurs seven times in a 19-year cycle, has 13 months instead of the regular year's 12. The added month is called "Adar I" and is inserted before the month of Adar (termed "Adar II" in leap years). This occurs in years 3, 6, 8, 11, 14, 17, and 19 of the cycle.
An interesting question. Here is what I wrote for the monthly newsletter at the congregations I serve. It is necessarily a very abbreviated response: there are entire books dedicated to the mathematics and explanation of this topic.
As I write this, we are moving towards the end of Shevat and the start of the month of Adar 1. It is Adar 1 (rather than simply Adar) because this is a ‘leap year’ on the Jewish calendar. There will be 13 months this year. The way this is arranged is that we insert a second month of Adar, so the first one is Adar 1, and the second is Adar 2. The reason for this is the re-calibration of the calendar, pulling a mostly lunar calendar back into alignment with the solar calendar.
This is necessary because a lunar calendar is 354 days in length, which is 11 days shorter than the average solar cycle. This difference is why we always hear (and feel) ‘the holidays are early this year’. In the years with no leap year, the Jewish holidays, though on the same day in the Jewish calendar, appear to be earlier on the solar calendar, moving as much as 33 days ‘back’. Then, in the leap years, the holidays are pushed thirty days ‘ahead’ on the solar calendar, so we hear it said that ‘the holidays are late this year’. Really, the holidays are always on the same date, but the correlation of that date with the secular calendar varies by more than a month, so it seems that they are ‘moving around’.
If no adjustment is made, the lunar and solar years would get farther and farther out of synch, and holidays would come 11 days sooner on the solar calendar each year, moving ‘backwards’ through the year. After approximately 33 years, a holiday would come approximately 365 days early, which would make them appear to be ‘on time’ on the solar calendar, but in reality they would be much earlier, a whole year earlier. That process would continue indefinitely. The result is that, for example, Passover, our Spring season holiday, could be at celebrated at ANY time of the year.
Side note: This is how the Islamic calendar works; the holiday of Ramadan, for example, is not fixed to any season, and ‘moves’ or ‘floats’ on the secular calendar from year to year. There is no adjustment made to the Islamic calendar to re-calibrate with the solar calendar.
To adjust the solar and lunar schedules for the Jewish calendar, a complicated formula is used which works in nineteen year cycles. Seven of the nineteen years in a given cycle are leap years in the Jewish calendar, and a 30 day month is added to the calendar in those years. That added month is Adar 2.
The adjustments are amazingly accurate. The calculations that date back to before the Mishnah (roughly 200 years before the Common Era) were really precise; the only thing not accounted for in them is the rotation of the earth and how that affects the calendar, introducing (as I vaguely understand it) an approximately 2 hour difference per century. I am not a ‘math person’ – and this degree of accuracy at that time in history is rather astonishing and amazing to me!