What caused the crust on on the top of my bread to separate from the rest of the loaf?

What caused it: it's gotta be gases from the yeast exhalations which are unable to escape the air-tight envelope of the crust layer. So they just build up in a bubble right under the surface. Cutting some of those large gashes or cross-hatches into the top of the bread before baking should break up the outer skin of the loaf to let some of that gas out without detaching the top of the bread.

I found with a loaf I baked yesterday in a bread pan that the crust not only separated from the crumb, but became wrinkly and flaky and fell off the loaf when cut. I think it may have been because I set the hot bread outside to cool on a very cold day, rather than letting it cool slowly inside.