My skimmer did that from time to time early on as well. As your tank ages this seems to slow down and eventualy stop. Has for me. Im like Jerry, I have no idea why but something you have to keep an eye on for a while. Id say around the 5 month mark was when my system lvled out and is predictable as far as running the skimmer wet or dry. I can set it to run black water or gray with out fear of overflow. Give it time.