There are possibly a list of reasons but I will answer 2 that spring to mind
1. If the A/C was live to help cool the APU bay (high temps)Temp sensors above aircon bay and beside the packs have been known to cause pack trips.
2. As hydraulic pressure bleeds off the speed brake is at the wilms of the elements (no internal locking in actuator)so even a slight tail wind can open them.