I could be wrong about this but I am of the opinion that whether or not the hair slips has a great deal to do with how the
hide is handled when first removed from the animal. If left wet for too long I believe that there is much greater chance of the hair slipping, but if the hide is salted and dried there is a greater chance that the hair will be set firmly in place.
Simply speculation based on what someone recently said in a thread I started elsewhere about keeping the hair from slipping on a deer hide- they indicated that soaking in a brine solution (before fleshing, for instance) will set the hair for good.
I would assume that the same is true for otters as well.
If anyone has evidence to the contrary I'd like to know!