What causes a cupcake that comes out of the oven looking awesome, with a nice dome, and the toothpick coming out clean, to "fall" as it is cooling?

Usually it's from being slightly underbaked. The toothpick test is just a helpful guideline. (But not always helpful.)  

Tricky situation - they looked great, tested great - I wouldn't want to have baked them any longer - I was afraid they would be "overdone".  Hmm.....

Thanks.