How do I reorganize windows in Exposé? Under Snow Leopard, sometimes a strange behaviour occurs in Exposé. I swipe four fingers down to activate it, optionally hover over another window, and swipe four fingers (I believe) again. Instead of closing exposé, the order of two windows change, with an animation.

I can't, for the life of me, figure out what I am doing to trigger this.

Any ideas?
The trackpad is not registering that second swipe as a four finger swipe.  Rather it is seeing it as a click-drag, and the machine believes you intend to reorder the windows.