Nasty rust spot on 98 Tahoe tailgate

I have this huge bubbling rust spot in the center area of my tailgate. It has been developing over the last year or so, is about the size of my hand now, and it is pretty ugly. The rest of the truck is entirely rust free, as I always keep it very clean.

I am not the original owner, but it does not appear that there was any body work done there or anywhere on the truck. I just can't believe that that area can go, in the middle of a panel with no visible damage like that. Anyone else have any experience with something like that?

Was originally going to have it repainted last year, but after seeing the quote I decided to keep my eye out for a nice used black tailgate instead!