Hello! OK, work with me here: Visualize you are getting ready to put fondant on your cake. You have your cake, it's perfectly iced. You roll out your fondant. You lift it and place it on top of your cake. Then you take your hand or grab your fondant smoother and start smoothing the fondant to the top of the cake.
THIS is a key time you make your filling bulge. If you are applying ANY downward pressure, you are pushing filling out the sides or displacing the air in the cake. You should only be applying a very light touch to get the fondant to stick to the icing.
You are not laying cement. :D
OK, assuming you are applying a very light touch, you start stretching and getting the fondant on the sides of your cake. Then you trim your excess. Now you take your fondant smoother and start smoothing your fondant, trying to make the corners even and sharp, your sides smooth, your top level...
This is also a key time you are forcing your filling to bulge. Again - you should ONLY be using a very light touch on the surface of the fondant, especially when working the top and the top corners. If you press down at all, you are forcing your filling to bulge.
This doesn't happen with ganache because ganache is so hard. But if you are working with any type of buttercream, cold or not, you can force bulges by trying to manhandle fondant onto your cake.