I'm lead to believe that solenoids sieze up, too.
I initially had that thought for the motor, too, until I got that sucker out of the door and into my hand.
They engineered it in such as way as to make it impossible to repair or rebuild. You cannot open it without destroying it. Cute.
Plus, a locked-up solenoid would be fairly trivial to bypass. This door was designed so that when the motor locks up, you cannot manually lock the door. That's just bad IMO.