They should only lose sanity from eating Strange meat and Strange meals (including Strange Kibble). If they are losing it through some other mechanism, it could be a bug.

I was also confused by this as my animals where restricted from entering my stockpile, i had some strange kibble saved up as a emergency measure. It turns out my tamer was taking it and feeding it to my pets. I put down a dog and panther that each went berserk and was briefly terrorized by my pet thrumbo. Great mod.