Was the water hot or warm? If you warm it up to much then it will fall apart. I tend to only warm mine about 30-45 min in a ziploc baggie put in water about 111*F. Then the rat goes from room temp to 90-95*F.

Oh wait!!! Did you thaw the rodent in warm water? If so next time let it sit on a table or something for like 8 hours and completely thaw at room temp. Then warm it up in >warm< water.