What causes flat spots in tread of trailer tires

[SammyChevelleTypeS3]

Just some of the many flat spot preventers for sale all over.... A few months sitting should not cause them. I still would not park a vehicle for too long under a heavy load.

https://www.amazon.com/s?k=tire+fla...9_13211746&tag=mh0b-20&ref=pd_sl_5svavangn8_e

 

[Traction]

Make sure one of the shackles between the front-rear springs isn't flipped on the pivot link on the inner ends of the leaf springs, which happened on mine. Struggled trying to get it back into position with no luck and ended up just pulling the trailer up a couple car ramps, heard a loud bang and it popped right back in place.

 

[JohnnyG]

Since this is tire wear and not from sitting (thanks for the clarification) it's probably being caused by severe alignment issues. I've seen it before on trailer tires. Spring shackles worn, or flipped as in the reply by Traction. Also check the U clamps that hold the axles in place. I've seen those shift and cause one axle to too close to the adjacent tire, which causes severe toe issues.