There are a few potential reasons why your dog's back legs might be shaving when they lie down. u003cbru003eu003cbru003eOne possibility is that they are experiencing some sort of pain or discomfort in their back legs, which is causing them to shift around more than usual. u003cbru003eu003cbru003eAlternatively, it could be that they are getting older and their skin is thinning, which makes it more likely for them to shave as they move around.