15" Marathons tread separation

I pulled one of the 15" Marathons from my TM because it was losing air (turned out it was a faulty valve stem). As I was rolling it into the tire shop, I noticed that there was a bulge on part of the tread. Not good. So I went back home and checked the other tire and spare. Turns out that one more of the tires also had tread that seemed to be separating.

These tires have maybe 7 or 8 thousand miles on them. One of them had been the spare and has only 3000 miles of wear. None of it in particularly hard or hot conditions.

I chatted with a friend who has a trailer rental place. He says that after troubles with Marathons and Carlisles, he switched all his stock to Westlakes and hasn't had any problems since. That's anecdotal data but all I have to work with. He sent me to his tire dealer and that's what I've put on the TM. A little heavier tire - Load range E, but load wasn't the problem. I suspect it was simply shoddy manufacturing.