If it's soft, the residual heat will heat it up while it's cooling. You must have started with it face down all the way to cool.

This actually prevents the steam from escaping causing all the moisture to build up underneath. You need to turn the cake upside down when it's still somewhat warm, and turn the cake upside down when it first comes out just to keep the top from collapsing and allow the crust to cool and solidify. Once the top has solidified, turn it upside down and let the steam escape from the inside.