Slippage of fur is 100% bacteria growth and bacteria needs to be at a certain temp to react to the flesh. Unless you took that fresh caught through the ice beaver, and put it under a heat lamp I highly doubt it to be slippage. Sounds like something wrong with the beaver probly when it was still alive.

Maybe you cut the hair follicles off when fleshing. Warm water will cause one to slip. Strange though. I like to use borax while im fleshing and it seems to help. sorry to hear of your problem. Good luck.