I have a groom's cake on Saturday for a friend's son. I bake all the layers and one of them has a problem. It is a square 13 inch layer and when I took it out of the pan and put it on the cooling rack, bottom down it got squooshy. The bottom seemed to slump out. I am thinking it was not done, even though the cake tester said that it was.
I am now rebaking that layer since it is the foundation and I'll make cake balls of the odd one.
Any ideas about what went wrong will be appreciated. Never happened before with this or any other recipe. TIA

It sounds like it wasn't done. That's my best guess. I have had a couple of cakes that came out with a clean toothpick, but when I leveled them, they were not done in the center. I had to re-bake them.