This just started happening to me the other day.
The game doesn't crash, it just gets minimized. I don't have to log in again, I just have to hit the ddo button in my taskbar to get back to playing. It only seems to happen while in a dungeon. It hasn't happened to me while standing around in a public instance.
Its awfully annoying, especially when soloing and in the middle of a fight.
I am not sure what is causing this. I have no programs running in the background. And its not my anti-virus kicking in.
Any ideas?