this is the 2nd time i've had this happen. the best guesses are that a newly mated queen went under the board upon returning & the bees followed her. the last time ended badly. the bees were using the comb under the hive for brood & the hive for honey. we were in a dearth & without thinking i opened the hive & set some frames of honey next to the hives & set off a robbing frenzy. it happened so fast & with the hive open there was nothing i could do but step back & stare at my blunder. my approach this time will be: put another bottom board next to the hive, put the brood boxes on the new bottom board, put an empty brood box on top, put the bottom board with comb on top of that, & put the lid on. hopefully the queen will move down into the hive. i'll check it after a few days. if need be i'll thump the bottom board against the empty box to knock the bees off the comb.