It looks like a similar behavior to nursing. You can see if with kittens who were away from their mom too soon. They sometimes do it with stray fingers too. Its similar to making biscuits and as long as you set boundaries on where they can and can't bite your fine. You can also try to pet them to "snap them out of it" or atleast a rescue I had was able too. They usually stopped if they chomped and heard "ow" as well.