That's one of the reasons a crust will go wrinkly.

Hi, it might be a shaping issue -- the surface wasn't pulled tight enough (happens to me way more often than I'd like).

Not enough information to answer the question ...Bread always shrink, or attempts to, when it cools.  Thicker or tougher crusts sometimes don't wrinkle or change during cooling, but thinner crusts do.  It's normal.  But how much is too much?  Got any pictures you can post?  Too much wrinkling can occur if you have large bubbles in the bread or if the bread is actually collapsing ...although collapsing generally occurs while it's baking, not while it's cooling.  I suspect your bread is normal, but a picture would be nice ...including a picture of a cut slice.

Brian