This happens to me on occasion and I have experimented with a couple of different techniques to try to prevent it but have not found the magic answer to fixing it. Sometimes I wonder if it has to do with humidity or heat.?

Does anyone know why this happens or how to prevent it??

Thanks!!

I sure hope someone has an answer for this one...my cake in the oven has a HUGE "dome" forming right now and I have no idea why...sometimes it happens sometimes it doesnt...

I've seen it happen with fondant cakes and from what I have been reading, it's from starting with a very cold/frozen cake and putting the fondant on the cold cake. From what I am understanding of it, as the cake comes to room temperature, gasses are released from the cake creating air bubbles under the fondant.
I've also read where the fondant cakes settle, which can create bulging sides.
I usually work with buttercream and I am trying to learn more about fondant. I have seen a buttercream cake, bulge, but very rarely.

HTH

A cake can bulge for several reasons. Putting fondant on a cold cake will cause air bubbles to form under the fondant. Another is not letting your cake rest after filling. I always let me cakes rest for at least 12 hours before covering or put something very heavy on top for several hours. A cake has to settle before it is covered. That's the reasons I have found with mine.

I use buttercream most of the time as well and the bulging only happens once in a while. It seems like the last time I tried to really press down the cake after filling and before frosting, I got the bulging even more. I am wondering if you really press down on the cake (putting something heavy on it), let it sit and then frost it if somehow it springs back up a bit and causes the bulging as well.? I am so guessing here but I am just trying to figure out why this happens so that I can maybe fix what I may be doing wrong.
I fear the day that I get a cake order for a very simple clean cake with maybe just a ribbon border around the bottom and then the bulge occurs and there is NOTHING you put on to hide it!

Are you doing a stiff dam of buttercream around your filling? I always do the stiff dam, fill my cake, and let it settle over night covered in saran wrap. I haven't had any bulging issues since starting this method.