Coincidence or Can a Brake Controller Activate Truck Brakes?

I agree with Gdetrailer as far as the pads most likely being stuck. At some point(you added the trailer) and the brake problem became coincidence. The pads were "stuck" and unable to release properly and wore down to where they are not a problem right now. Most likely the pads and pad hardware need to be removed, mounts cleaned of rust and I also like to put a dab of silicone under the hardware to slow down the rust build up.