I had the same problem, but only in one hive, the strongest one. I fed the same sugar to all five hives, but it happened only in this one. Weather is horible here, but I had a chance to look inside before and after this had happened. Everything was OK, a lot of brood the queen was fine. I thought that may be for they are so active, have plenty of brood to feed, and eat a lot, and the weather keeps them iside, they just make a quick "walk" outside to clean themselves, and go back.?
mat