It was probably the witch hazel, I avoid using it as much as I can. And if it was raining the humidity will affect them too. I always check the weather before I make mine. I swear they a re weather contingent. If you put any epson salt on them even as decoration on the top, they will also suck the moisture and crack. At least this is my personal experience.
As an option when your bath bombs go wild, I pack them in cute bags. I personally make embeds for my bombs, so I add an embed, and maybe some colored epson salt, or little flower buds, or chamomile and pack them in a bag, like in this picture.