When you have a tandem trailer and turn it sharp, one of tires on each side has to skid sideways a little to make the turn. (This is why it's next to impossible to turn a tandem trailer when you push it by hand.)
Depending on the load on each tire it may skid the front or the back tire while turning and could be a different tire in different situations. This usually happens while backing up, because you tend to turn the trailer sharper, even pushing the trailer tongue almost sideways if you're really cutting the trailer sharp. Look at the inside tires as someone else backs up into a driveway and you'll be amazed how much one of the tires skid sideways.
I had a bass boat that I would pull up past my driveway, and then back into our driveway and then garage. The driveway was a slight incline, so as I cut the trailer into the drive way the rear tire took most of the weight and it skidded the right front tire sideways as I turned.. After a year with dozens of fishing trips my left rear tire looked just like the one in your second photo. As the tire repeatedly skidded each time I backed into the driveway it flexed the tire sidewall and rolled it over a little and scrubbed the tread off the outside of the tire. After only one year here is my tire.
View attachment 31095
I had the alignment checked and it was fine. Took me awhile to figure out what was happening.
I have no idea how often you trailer your boat or if you routinely back up in the same situation over and over again that could cause this, but it might be something to look into.