Plymouth, Massachusetts wasn't the planned destination of the pilgrims. In the charter they signed with the Virginia Company in England, they agreed to settle on land south of the 41st parallel locted near the Hudson River. The generally accepted reason for their sailing north of their intended destination was due to stormy weather, however there has been some unsupported speculation that they may have intended to ignore the provisions on the charter.
Read more at: www.TheMayflowerSociety.com