This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
From J.COOK@ens.prime.com (Jim Cook):
Black powder motors tend to suffer catos when they are temperature
cycled. If you expose them to heat, be it storing them in the attic,
on your car's dashboard, or in your metal range box in the hot sun on
the launch field, you may have problems. The engine expands with the
heat, but when it cools, the propellant separates from the casing
inside This causes the propellant to burn faster due to burning on the
side generating more pressure than was designed for, and ...boom...
Storing black powder motors in a damp basement can cause the compressed
clay nozzles to soften and also blow out. If you must store your motors
in a damp/humid area, put them in a zip lock plastic bag.
[Note: There is an excellent article by Matt Steele in the May/June 1992
issue of Sport Rocketry. This article goes into the
theoretical reasons why black powder model rocket motors fail]