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.