My bees did this a lot when I first got the hive and it freaked me right out! I was told to just be sure the queen wasn't stuck under there in the middle of them and otherwise it was probably normal, but may be because they are too warm inside the hive.They still hang out there some but not so much now. We made sure the queen was topside and I did prop my cover up a little to try and increase ventilation a little in case they were hot. After I did that the number decreased. Removing the mite board will probably do the trick. :).