Think of it as too much metal in the middle of the panel. It can't move sideways so it bulges out. You need to "shink" the area in the middle of the panel.

I can tell you that it can be done with a torch by heating small spots cherry red, then quenching with a wet rag. I am not a "metal guy" so I can't give you specifics. If you have a good metal worker in your area, you might consider taking the door to him for repairs.