I saw this at TRU. I'm pretty sure it's the bronies, at least in my area--ALL of the MLP blind bags (2 boxes) were opened, but no other blind-bag item from any other line (and there are a lot out right now, including Lego minifigs, Playmobil figs, and lots of other big-name stuff) was damaged in any way. Only 1 or 2 ponies actually stolen out of all that, too, and the bags were too high for tiny tots to reach, so it couldn't be blamed on the target demographic.
I've been thinking of starting a Pony Tales RPG campaign using the teensy little guys, so I've decided to buy ANY opened blind-bag I see, regardless of what's in it. (Though it would probably be a lot cheaper to get the multipacks...)