Does anyone know why the flight returned to IAD?

http://flightaware.com/live/flight/U...208Z/KIAD/KIAD

I have a friend on the flight, and he says they were give very little information. Also, as a non-status passenger, it sounds like they had to wait in a line for hours as two employees dealt with hotel & food vouchers.

While his resolution seems to be to never fly United again, I'm more curious as to what happened to the plane.