I don't know much about animals, but according to this website, it could be because of two things: 1. If they salivate while being petted, it is likely because they are happy and relaxed. They might associate your affection to what they felt when their mother nursed them. 2. If they salivate without being petted and if it is new behavior, they need to be treated by a vet. It is usually a bad sign. Refer to the website for more help. I hope she'll be okay. Cheers ✌️🏾