Our 1977 Jeep Cherokee, I was told, was not able to be adjusted for camber. As a result, the tires wore badly on the outside shoulder. While on a long trip I stopped at a wayside and observed the same thing, only all around the tire and the shoulder was worn badly. I replaced it with the spare.
My fibers were shiny from hitting the road.
Yours looks like a manufacturing fault unless you overused the tires. See below. It may be that overheating would allow the I would take them back to the place you bought them and ask for credit on new ones. Of course then you will have to buy the same brand.
I found a good place to buy tires is Les Schwab. They have shops all over the west and they stand behind their tires. (No, not because they are afraid to stand in front!)
Were you running them at the 50psi? At that pressure you have 1360 x 2 = 2720 lbs total capacity. Lower pressures require lower weights.
How fast do you drive? What comes after the "ST175/80R13" s/b "91L" 91 means the load rating you quoted and the L indicates the tire can run at 75 mph. If the letter is earlier in the alphabet, you may have a problem; K=68; J=62; G=56; F=50. Since you live in CA, you may have G rated because trailers are not supposed to be towed faster than 55mph. Ahem.
Here is an unbiased discussion: