My horse does this too. He then comes to the gate to wait for me. I believe he does this because he knows he is going to work, and is getting ready.
Don’t knock it – he never pees in the trailer or crossties because he takes care of things himself at the start. Before loading to return, or putting him in the crossties after work, I always walk him to a nice soft spot, and he knows to pee. I taught him a command to help with this – just picked a signal, and started using it and praising him when he urinated. It really comes in handy!