Probably a bad ground in the brake light (either right or left) backfeeding through the ground in the rear window defroster and turning the light on!
Where ever that ground connection is, you will probably find the defroster ground and brake, tailight, backup, ground attached at the same point. Cleaning this connection and re-tightening it will probably solve your problem.
Why it doesn't do it when your sitting, I can't say, but if the ground goes away, the circuit will try to complete the path any way it can!