:S Uff da as we Scandinavians say.
Wow, that is an ugly bulge in that tire.  Wow again!!!
I once owned a 2003 Chevy dually, with those same tires on it (Goodyears).  
Airing them up one evening, getting set to go camping and the fronts had a serious crack/split in the sidewall all the way around the tire.  This was a deep, serious split!  
Replaced all six with Michelins and never any more problems.  
Back to the bulged.  :h  I too do not see any other marks on the tires, other than at the bulge point.  Strange situation.  
Glad that your other tires are wearing well for you without problems.