Dunno. Termites. May want to collect some and take them to your local county extension office to find out. We have a severe problem here in NC with a pine beetle that is killing pine trees. Don't know if you have it out there.
If it's what I think it is, it's mites of some variety. My guinea pig as a kid used to get them. Are they tiny, tiny? White? Appear to jump around? If so, mites. I mix my pine shavings with DE to prevent them from getting on my chickens. I don't know if they'd bother the chickens, but they bothered my guinea pig as a kid, so I err on the side of caution.