I've had racoons climb a single antenna pole, to enter an attic vent, I saw a cat run up a 9 lite utility window, I have Blue Bird houses and they're always being tormented by something. I'm betting racoons, are your culprit. If you can, set up a trail camera and the mystery will soon be solved. You may want to set up a multi strand electric fence. Raccoons are very smart, strong , and can be very heavy they may be climbing up and just their weight could cause the hives to tip over. Good luck.