What caused George Washington to change his birthday to February 11?

Because England had not adopted the Gregorian Calendar when he was born, Washington's birthday was recorded as February 11, 1731 (under the Julian calendar, the year began on March 25).

Adjusted to the Gregorian Calendar, it became February 22, 1732.

Great Britain adopted the Gregorian changes in 1750, with the result that September 2, 1752 was followed by September 14, 1752.