To add to the discussion, I often bake, fill and ice a cake the same day and I don't get this bulge. Here is my theory on it.
I found out one way to get a bulge, quite by accident. Only once did I let a cake cool completely while it was lying with the unlevelled crown face down on a cooling rack. Now generally I level my cakes as soon as they come out of the pan, I don't wait until they are cool because I find this easier. Well it is easier with from scratch, not with cake mix cakes which I find generally are more delicate and this cake I had the problem with was a cake mix. Of course what happened was that the cake tried to level itself while cooling and this is where the bulge came from. So I won't be doing that one again. It was just because I had used all of my cooling racks and didn't have extras to flip the cake over so the crown was on top.
But I don't get a bulge even with filling and frosting a just baked and cooled cake. So my theory is this. Most people when levelling a cake, only level off the top bump. To truly level a cake, you want to bring the cake down level so that where the top meets the side is exactly the same height. And you want to use a really stiff buttercream damn and have your filling slightly below this height. If your filling is really of a wet unstable nature, well for one thing, it can absorb into your cake. Now to me this would be a special issue with a freshly baked cake that hasn't had time to seal itself by sitting on the counter covered for a day or so. Maybe the reason this isn't an issue for me is because I use a thinned apricot glaze to seal my cakes.
Anyway, it is an interesting problem. I tend to also fill in with icing at the seam as I am crumbcoating or frosting the sides of the cake. I guess this amounts to the same thing as putting an extra row of icing there.
Hugs Squirrelly Cakes