I am having the EXACT same problem. I am running an app, everything's running smoothly, then WHAM, back to the main screen, or WHAM, into a wholly different application.
The only time I've ever been able to replicate this problem on demand is using the maps feature. I enter in a start and end point, go to the map view, and moving anywhere off the default field of vision (zooming in or out, or moving in any direction), it does it.
So I made 2 videos, one shows the drop to the mainscreen, and the other one depicts jumping into my contacts list. I know the videos aren't spectacular quality, but I assure you there's no trickery and I did try to keep the entire phone in the cameras' view.
http://rapidshare.com/files/42218531/DSCN1404.MOV.html (7.6MB)
http://rapidshare.com/files/42218533/DSCN1405.MOV.html (13MB)