While I agree this was probably just a shoddy continuity error with the special effects, I have to suggest that most people, having just noticed flames shooting between their feet, might be likely to move to one side.
The original version of the movie already has Doc move, but when they put it in theaters, they took it out considering it was a waste of time to keep it.
So you are saying they should have actually set the actors feet on fire in this scene just for the effects to work. I know the actors got paid a lot for this film but I don't think it was enough money to have their feet set on fire!