ISO: Angel Food Cake Flop

I'm hoping someone can shed light on my problem. I've always considered myself an experienced baker, but this week I had a real flop. I had about 12 egg whites in the freezer and decided to make an angel food cake. I followed the directions precisely (and of course did Not grease the aluminum pan and the pan was Not non-stick). I was feeling pretty smug because the batter hardly deflated when I folded in the flour, sugar and cocoa. I baked it according to directions and when it tested done, I inverted it over a funnel. The cake rose so beautifully that it actually came up to the level of the legs on the cake pan so I had to use a funnel. While it was cooling I left the room for a few minutes and when I returned the entire cake had dropped out of the pan (including the tube part) and flopped onto the counter. It was completely cooked but we ended up eating it in chunks. Also, it was not a humid day at all. I live in Central California where it is dry and hot this time of year. Any thoughts out there?
Thanks.
Sincerely,
Marilyn