"A graphics problem has been detected"

MacBookPro11,3

2.5 GHz i7

Intel Iris Pro and NVIDIA GeForce GT 750M (auto graphics switching on, it was on intel) OSX 10.1

The system appeared to freeze. iStat Menu's clock stopped, the cursor wouldn't move, etc. I tried (physically) closing and reopening, but it didn't go to sleep and the apple logo light on the back stayed on. Option-command-power and option-control-power also didn't do anything. After force shutdown (long press of power button) and startup, it asked if I wanted to reopen Finder windows after force quitting it, "you shut down your computer because of a problem", and:

There's not much I was doing, there's not that much I have installed (relatively new computer). I'm guessing it was Focusrite's Saffire Mix Control. Or maybe Safari.

What could the cause of this be?