I'm running 2 GTX 560Ti in SLi (Multiple Display) and this has been happening to me quite often, although not 100% of the time... I tend to find that if I wait a minute or two before tabbing back, the game doesn't crash. I don't know if it could be something like the client having to initiate some sort of sleep-process to reduce CPU/GPU usage whilst minimized but we are trying to tab back in before the process finishes? It's probably not even a real process, but seeing how others are having this problem with different hardware to me, I feel more inclined to believe it to be a software issue.