AEG Lavatherm 57700 Condenser Dryer?

I have replaced the motor, pulley, belt and the front tube felt bearing.
I have put it all together and started the dryer. The problem is when the dryer motor reverses the belt wants to run to the right side of the pulley-- toward the door end of dryer.

When the dryer is running forward the belt stays in the middle of the pulley like it should.

It is only in the reverse that is runs to the right side of the pulley. What is causing the belt to run to the right side of the pulley?