If you used someone to pick up or deliver food or used a different vehicle that is not registered on your GrubHub account then it might be the reason. Could also be a customer reported that someone that isn't you delivered the food.
The best thing to do is to wait for their feedback, some users have had their accounts reactivated after review. The last option, if you don't hear from them for about 3 weeks, is to file a lawsuit against them if they do not provide actual evidence to prove their accusation.