Can anyone tell me why cupcakes flatten after they are out of the oven for a few minutes.  They are very high when they come of of the oven but they flatten.  Made with box cake mix.

You could try cooling them on their sides.  I know that some cakes are cooled by inverting them because the weight of the cake will cause it to fall.(Angel food cake comes to mind)  So try to reduce the pull of gravity a bit by cooling on their sides. (Very hard to invert a cupcake but if you can maybe that is the way to go)

I read somewhere that over-beating the batter on high speed can put too much air into the batter, which makes cakes/cupcakes rise higher when baking, but makes them fall when they cool.  Not sure if that's true or not, though.