The blend door is used to switch the HVAV between hot and cold, and the motor cycles it back and forth to find the max position each time you start up the heating or AC. Basically, it moves the door as far as possible until the motor stalls and then stops. This puts pressure on the small gear that attaches the motor to the door pivot. Usually the door pivot (which is plastic) eventually fractures.
This issue has been a problem on F150s, Expeditions and Explorers also. Its expensive to fix because the door is buried deep in the dash plenum.