i just ordered some mice from bigcheese, guy told me they lost 70% of their rats from some kind of 'temperature malfunction'. if that shop was buying from bigcheese, that's probably why.
I've had that happen to me twice now with my own small rodent colony. If the circuit blows on the exhaust fan the temperatures can get ridiculously high in a very short time. Nearly everything in the upper tubs of the rack were lost while the lowest tubs were okay.