It's because of the moon.
From Wikipedia
Easter is observed on the Sunday after the first full moon on or after the day of the vernal equinox, this does not reflect the actual ecclesiastical rules precisely. The reason for this is that the full moon involved (called the Paschal full moon) is not an astronomical full moon, but an ecclesiastical moon. Determined from tables, it coincides more or less with the astronomical full moon.
The ecclesiastical rules are:
Easter falls on the first Sunday following the first ecclesiastical full moon that occurs on or after March 21 (the day of the ecclesiastical vernal equinox).
This particular ecclesiastical full moon is the 14th day of a tabular lunation (new moon).