This has happened to me on my last 2 or 3 cakes. Here's my process: I crumb coat and put in fridge for an hour or so. Get my fondant ready. Pull out the cake and lightly coat with water so fondant will adhere to buttercream. I cover and smooth it. It looks great. Then I let it sit for a little while and I get big air pockets trapped between the cake and fondant. How do I avoid this? Any help would be appreciated.