The door mechanism is motorized. The mechanism itself is attached to a push rod that handles the opening function.
There are a number of potential problems that can cause this:
1. The power supply feeding the door mechanism is burnt out.
2. The motor handling this function is burnt out. This usually occurs when the door belt starts to slip on these units. The limit switch for the doors don't engage when they should, and the motor runs longer than it should.
3. The motor controller board has a problem, due to #2 above.
You should get a set of doors installed to actually test the doors under normal load. The glides will appear to function properly, but that may be just an illusion since the normal load is not on the belt and the motor.
B&O should offer preventative maintenance on this sort of thing at some reasonable cost, when units get to be about 15-20 years old. Virtually every unit out there will start failing when the door belt is stretched too far, or the limit switch is not engaged properly.