Could be caused by low memory, after market launcher/lockscreen, or ROM (if you are using)
Like others have mentioned, it is Sense crashing and restarting. On my Rezound it usually happens after exiting an app that uses lots of phone resources like some games. Another possible cause is one of the task killers, if they kill some processes that are critical to Sense it will crash.
It only happens occasionally on my Rezound. Just before I upgraded my Incredible, it was crashing with EVERY app. Those last updates made it almost unusable.
Johnie