This past week was really warm/humid for this time of year. I made 2 cakes that evening (both covered in fondant), let them sit over night in a really cold, air conditioned room & that morning it was freezing cold so I turned the AC off & when I went check on them I noticed they were sweating & there was a huge bulge poking out on both of the cakes. It looked like a big bubble/egg coming out the side of the cake, but when I pushed on the fondant it felt hollow, I didn't feel any filling or anything inside. It never busted, but I was so worried it would before my customers came pick it up. Luckily the bulges were in the back of the cakes & everything came out fine, but I'd like to know why this happened so I can prevent it next time.

What could have caused this? The weather/temps maybe?

Hii there, i think the problem was with the change of temps,( if its very very cold the room and its warm outside thats whats going to happen) . I live in Puerto Rico and its very difficult to work with fondant because of the temperature. Take Care .

Next time just take a pin and prick the bubble, it will deflate. Your room was too warm and firmentation took over. Bet you had fresh fruit in your filling. I was just at the New Jersey show and in talking with a big Food Network Cake star, he said to put the fondant covered cake in the fridge over night, wrap it up in saran (once it is cool). When you take it out of the fridge drape a hand-towel over the saran draped cake. Moisture forms on the outermost surface, hence the saran and the towel will absorb the moisture off of the saran, then just remove the towel and saran. I asked him if the saran would stick to the fondant and he said "no". I need to test this out myself but that's how he does all his cakes.