wtf did you hit…big ass pothole?
the usual cause of that is an impact to the tire that causes a small break in the inner lining of the tire, which is what holds the air inside of the tire. once that lining is punctured, you get the big bulges where the breaks are. if yours kept expanding, chances are that break kept getting bigger and was damn near ready to pop sonnnnnnnnnnn.
these breaks are most often NOT the result of a tire defect. a defect usually manifests itself in broken tread patterns aka shifted belts, not an expansion like this.