Check your trailers up-travel and tire clearance.
If the alignment is good and the axle isn't visibly bent,
I'd bet that tire is contacting the trailer chassis/fender under load and SKIDDING on the asphalt momentarily.
You likely won't feel the skidding because the bump that caused the compression also causes the trailer weight to unload immediately after.
I have a tandem axle trailer that had that very issue.
I flat couldn't figure it out for a couple of years.
I cut off all of the spring mounts and started over with new springs, hardware, brakes and axles.
The Problem persisted.
This is the only trailer I have ever owned or dealt with, hell, I just figured 1 new trailer tire per race was going to be part of my prep.
It had never "failed", so I just replaced 5-6 new tires per year, 1 at a time.
I loaned the trailer to a friend for a move from San Jose to LA.
He loaded it "Far Heavier" than I ever did,,,,,
The axle, along with the Brake, wheel and tire BROKE OFF at the spring perch U- Bolt, somewhere between Grapevine and Magic Mountain
on HWY 5.
Never to be seen again.
Apparently, the constant rubbing was enough to fatigue the axle to the point of failure.
The problem became obvious once I saw the carnage and the actual failure of the axle tube.
Sure glad it wasn't me who had to 3 wheel an 11,000 lb trailer/load from Magic Mountain to Disney Land, on surface streets!
I reworked the fender mounts, replaced all of the suspension, AGAIN, and haven't had a bit of trouble since.
Yup,
Cool Story Bro!
Seriously,
check your up-travel clearance.