random trailer brakes locking

[Izzys11]

2012 freightliner with a 2000 sun valley trailer. while driving in traffic around 20mph trailer brakes have locked up on 4 different occasions.
i was not touching the brakes when they locked up, air pressure was full on both tanks, no oil or water in tanks .It is an instant lock when it happens but a slight tap of the brake pedal will release them. Four way valve on trailer was replaced along with the emergency valve on trailer. Went a couple thousand miles with no issue then it randomly locked again and instantly released with a tap of the brake pedal. Any ideas ? Maybe coming from a valve on the truck?