Trailer brake and ABS

[Starlight]

Ah ha, very timely. I have a peterbilt at my shop right now with that axact same problem. I found the drive axle relay valve defective causing its crack pressure to be about 40 psi on the gauge before the brakes applied and then they applied hard. Crack pressure on this paticular application is supposed to be 5.5 psi. Have a look at that on your trailer. The abs valve which acts as the trailer relay valve as well might be your culprit. But confirm first that your control line has no leak as that could cause an issue like this too.