What causes this?  The icing poofed out... it wasn't that way until I moved the cake to the car today.  What did I do to cause this?

I've had that happen to me also.  Wonder what makes it do that?  I take a toothpick and open up a little hole and that takes care of it, but what makes it do that in the first place.  It is usually on smooth cakes.

When you finish your cake on the cardboard, you should "break the seal" at the bottom of the cake, to allow any trapped air to escape.  Do that by running a thin blade around the bottom of the cake.  That's what we were taught in culinary school.  OR poke a hole in the top of the tier that will eventually be covered by another tier or a decoration.

Thanks, I'll give that a try.