Maybe they put bunny in the regular hold instead of the pressurized area? A bunny autopsy is in order.
The whole fuselage including cargo hold is pressurized, but it can get pretty cold in some spaces below the passenger floor.
Stress and being frightened in a cold cargo bay can kill a rabbit quickly.
I would never put any animal of mine in a cargo hold, especially a rabbit.
Every cargo area is pressurized. Depending on the aircraft type some are heated and some are not. They also have fire detection and extinguishing.