I freeze my cakes and decorate them while still frozen. It's always worked out easier, but lately, I've noticed that when I try to ice my cake, the sides of the cake start falling when I start crumb coating. It'll first start to puff out in the middle, and then will eventually come off when I try to smooth over with my spatula. It's to the point where the icing and some of the cake can peel off in huge chunks.

Any ideas why this is happening?

This is exactly why one should NOT ice a fzn cake - cold yes, fzn NO. And it should be crumb coated before you fz it.

Emfledd, I have often iced cakes while they were nearly frozen. Lately I am having the same problem. I am thinking like the others who have posted, it's condensation.
Summer time humidity is not helping.
I see you are in south as I am. Summer time humidity does not play fair with cakes.

I torte my cakes and let them sit for a few minutes to warm up a bit and this is helping.
The condensation is keeping me from smoothing with a viva paper towel too. The cake is too wet.
I have tried tweaking my icing but the best results I am finding are by allowing the cakes to come up to temperature.
An icing bag, as seen on the 'Cake Boss', helps for those of us who have heavy icing hands.

Awesome!
I was thinking that the reason was the cake thawing & the summer humidity is certainly not helping! I'll definitely change my baking habits now!

Thanks everyone!