I had not considered that a belt might have broken and resulted in a section being pushed out. The tire certainly is out of round, and very obviously so.
I'm getting new tires tomorrow (all 6), so we'll inspect the bad tire and see if we can determine the issue. These tires are Toyo AT-2, and they've been excellent otherwise. 50K miles on them now and still about 20K miles left on the tread wear. I'm going back with Toyo's, but this time I'm using the new commercial-tread tire which has stronger sidewalls. We commonly use this truck to haul the gooseneck mentioned, plus a 16,000 backhoe. Overall, the weight is somewhere around 22K to 23K, so it wouldn't surprise me that a tire(s) might fail with those kind of loads.