Sometimes, things are just a mystery and we'll never know why.
My Outback was going on 2 years old (original tires) when I noticed one of four tires had very uneven worn tread and the other 3 did not.
I posted about this on these forums and got all kinds of sage advise from replacing tires to having my axles examined for correct alignment, to a whole bunch of other things.
I never did figure out what caused but had a theory.
All 4 tires were the original tires from the manufacturer. I don't even remember what they were now. But quite obvious, they were not high-end, high-quality tires for sure. The best I can figure, one tire was more defective than the other 3. It simply was a bad tire from the very beginning, or manufactured without good inspection, or who knows. Anyway, it was just not an equal tire as the other 3, and the rubber wore uneven.
Maybe the position of tire had something to do with it? It was the passenger side front tire (trailer), that also was more prone to scrubbing curbs, making right hand turns (which I highly avoided doing). But maybe something about the turning caused that specific tire to wear more.
Anyway, I did not follow the sage advise, and just got all 4 new tires (Carlisle's) and its now been 3 more years and all 4 of my tires are not showing any tread wear. At least they all 4 look equal, and the tread is equal. So it wasn't a bent axle or anything out of alignment. I think it was just a funky tire.
Maybe your's is just that simple too.
Good luck. Hope you can resolve it as easily as I did mine.