My guess is that it might have been electrical, and possibly their KERS system. They had problems with it before, and if the car was live, as has previously happened, then Vettel could have been killed.
I can't think of anything else that would be an emergency that needed him to stop right away. Oh yes, there is the old wheelnut-coming-loose, or the fuel-leak-from-punctured-tank, but I still think it might have been the KERS.
My guess is that it might have been electrical, and possibly their KERS system. They had problems with it before, and if the car was live, as has previously happened, then Vettel could have been killed.
I can't think of anything else that would be an emergency that needed him to stop right away. Oh yes, there is the old wheelnut-coming-loose, or the fuel-leak-from-punctured-tank, but I still think it might have been the KERS.