This happens when you kill a process (or it fails) that is being used by the UI. So if you are running the calendar widget, for example, and you autokill the calendar app, you will trigger a UI reboot (which will just relaunch the calendar app for the widget, so don't bother autokilling the calendar if you want to use the widget!).
For wryun, I'm guessing Handcent was interacting with a widget? Like perhaps the messages widget? Perhaps you had a conflict where both the stock messages app and Handcent were tapping the widget? Dunno about that one -- sounds like instability in Handcent was as far as you can go with it. I'm running Handcent and have used it to replace the stock app for the messages widget, and I haven't had a single crash or force close.