The first answer which I cannot help but give to this question is quite simply that it isn’t. As is the case every year we will be celebrating the festival on the 15th day of the month of Nissan, which this year begins at sunset on March 25th.
However, I understand that with Pesach falling in the month of March it feels like it is early, which needs some explanation.
The Jewish calendar follows a lunar cycle. This means that at the start and end of the month the moon is at its smallest in the sky, while in the middle of the month is the time for a full moon. It is no coincidence that Sukkot, Tu Bish’vat and Pesach fall on the the 15th day of the month, when the moon would be at its brightest.
There are still 12 months in a lunar year, with months that last for 29 or 30 days. This means that every lunar year is 11 days shorter than a solar year. If there was no corrective for this discrepancy then over the years the lunar and solar calendars would drift further and further apart, creating a seasonal shift in the times when the months occur. This is what happens in the Islamic calendar; so that festivals celebrated by Muslims move throughout the seasons, moving through our secular calendar.
The Jewish calendar cannot allow itself to become out of sync with the seasons because of the agricultural and seasonal nature of the festivals. Sukkot, Pesach and Shavuot were originally harvest festivals.. They make sense at specific seasons in the year; celebrating the fruit harvest of Sukkot in December or January would make no sense.
The Rabbis therefore introduced a leap month as a corrective to ensure that the calendar never drifted too far away from the seasonal cycle. Over a 19-year cycle there are seven years with an additional month; these are in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years over the cycle. The additional month is Adar sheni (the second Adar), which happens after the regular month of Adar, usually between March and April. Adar is the twelfth month of the Jewish calendar, and so the additional month comes at the end of the calendar.
We are currently in the Jewish year 5773, which is the 16th year in the Jewish calendrical cycle, which means that next year will be a leap year with an additional month. This means that Shavuot, Rosh Hashannah, Yom Kippur, Sukkot and Chanukah will all feel early over the next several months, but by the time we get to Pesach in 2014, in the Jewish year 5774, it will feel like the calendar is back to normal.
One final thing, as a result of these unique calendar patterns the first day of Chanukah this year will coincide with Thanksgiving. That is the earliest day on which Chanukah can fall in our secular calendar, and it is the latest date on which Thanksgiving can be. Due to a number of other calendrical issues (more than I can deal with here), none of us have experienced this before, and none of us will experience this again.