random trailer brakes locking

[JimT]

Rawze's suggestion should be looked at first. It's both the easiest to check and the most dangerous if left uncorrected. But here are some thoughts, should that not be the cause.

That also sounds like the clueless handiwork of those early automatic emergency braking systems, especially when in traffic. If you have one of those forward radar/collision mitigation systems then it could be the source. Those old ones were very prone to misalignment and malfunction. However, if it was due to misalignment then tractor and trailer brakes should be actuated then should release on their own. Malfunctions should cause codes to pop up, but not always.

And that also ties in with the ABS, as do roll stability systems and traction control systems. Because of these safety features the ABS system can actually activate or release braking on any wheel. At least on the tractor, the trailer might be an all or nothing application as directed by the tractor ABS. Which means there could be an ABS issue, possibly on the trailer, or more likely with the system on the tractor.

Finally, while I'm no expert when it comes to the air system on a truck, i assume it's possible one of the brake valves in the tractor is failing to close completely and leaking air into the trailer service brake line. This might actually make sense if it only happens when driving in traffic where you are lightly on and off the brakes a lot.