What would cause...

Just curious. When you say "outside", are you referring to the outside edges of the tires, or the outside of the trailer? IOW are both edges of the tire worn?

Overloading causes the tire to wear the inside of the tread if all the suspension components are in good shape .....as in not enough camber bow.

Wear on the outside of the tires tread can be a toe problem as in a bent axle fore or aft.

One of my drivers reported sever wear pattern like that on a 34' GN trailer. On his reload run I crawled under neath for a check. I found both axles bent with big scrape marks on the bottom/center of the axle tubes. The driver had drug the trailer over a tall cement roadside devise hidden in weeds and bent both axles enough to cause severe outside tire wear.
A two hr trip to a big rig trailer repair shop to have the axles bent back in alignment got him back on the road.

Its odd that all 4 tires are wearing the same.

Or if the axles are completely upside down. Then the arch would be opposite what it should be causing wear on the outside of the tires. You said they came loose and rotated?

We took them off a couple years ago and brought them to a shop to replace the four tires.brought them back home and installed them again and that's the last I had done anything with it. I left maintaining them to my father. Unfortunately he isn't the greatest at checking the pressure and I'm sure that had something to do with it.
I dismounted all four and spun them around and then pressured up to 90psi. They are F rated and will go to 95psi.
A combination of under inflation and suspected overloading, (I'm always telling mom to leave a few more things at home) resulted in a slight wear on the outside rib of the tire.
We will see what happens this season.

Maybe the tire shop put the jack in the middle of axle?

If bought used, then could be improper wheels for the application, i.e. wrong offset perhaps.

MFL,
You are correct. This happens, strange but fact. Happen to me. After all the others, manufacture of trailer and axles, said it was bent axle or spindle and little flat bed trailer builder told me to replace the tire. I did, no more wear.

Donno128,
A grossly overloaded trailer diffently will cause tire wear BUT on the INSIDE. Not on the Outside

Take it to a truck suspension shop and get it aligned.

donn0128 wrote:
grossly overloaded, .

Would that not move the load pressure to the INSIDE of the tires ??

How many miles and what age for the tires ?
And how bad is the wear difference ?

IF....the tires are over-rated and thus lightly loaded, relatively, and they are run at pressure that is too high......some extra wear on the outside edge might be "normal".

I would start with checking air pressure. If not at max pressure, could be cause. Could be worn bushings/shackles. Sometimes people just replace the tires, and never any wear issue after???

Jerry