I had a similar problem, exactly as you described. Only, mine was the passenger side front tire. I also asked the same identical question on these forums that you just did ... exactly.
I got lots of responses about bent frames, hubs loose, miss-alignment, all sorts of things. I got recommendations to take it to trailer shop and have them analyze the alignment of the axles.... well, just about everything you can imagine. The trailer was 2 1/2 years old. But the tires were also the originals that came with the trailer.
What was weird was the fact I checked my tires routinely as I put chocks under them and remove chocks all the time, every trip, every encounter. Then, all of a sudden, half the tire was worm flat. Very weird, very strange.
Well, I didn't take the advise of anyone on these forums after all. My absolute "gut" feeling was based on an inferior tire. Three of the tires were holding up pretty good, the forth might have been a factory fluke, or as some say, "cheap Chinese tire."
I did examine the frame and did my own set of measuring, and nothing appeared out of the ordinary. So I had to get a new tire anyway, so replaced all 4, even though only one was worn bad. I went with Carlisle Tires (the new version).
Now that was 3 trips from Indiana to South Carolina and several thousands of miles ago when I made the tire switch. And to this day, I'm not seeing any signs of uneven wear, and I'm a little more meticulous now, using a tire tread gauge and comparing tread depth with the other 3 tires now. All are wearing even, across the entire width of the tire and so far, no noticeable difference.
If the original was not a cheap tire, then the only other thing I can think of that caused the apparent change to occur so fast was tire pressure. I will admit, when I went on that one trip and discovered the worn tire, I had not check the tire pressure in a while. It's possible the tire pressure in that tire lowered which somehow might have caused it to wear different than the other 3. At that time, I was also keeping 55 pounds in each tire. The new ones, I'm maintaining 80 pounds now (the max pressure). It's possible, the lower pressure could have caused the uneven wear.
But one thing I am convinced of, it had noting to do with the trailer alignment.
