I'm just pointing out that the argument that Pope as Police Chief wouldn't bring a lawsuit on his own department doesn't wash, because the machinations that set the lawsuit in motion started before Pope had any idea he was going to wind up being chief. In fact they started right around the time Pope's career was looking like it was at a dead end, to him. I do think it's feasible that he had a short span of irrationality, which is all it would take to set things in motion. Maybe he thought better of it all later, but once the wheels were turning it would be impossible to make them stop.
This got me thinking that the clues to the mole were in season six and season seven was filled with red herrings. Luckily they were reshowing season six on OnDemand at that time. I found the most important episode to be the one were Brenda shoots the mad bomber, and loses any chance to become chief. At the end of the episode Gabriel thanks her for saving his life, and Pope says he's going to update his resume. I think at this point Pope was extremely bitter, at the department in general, and specifically with Brenda. He also wouldn't be concerned with ruining his career, since he thought his career was already ruined. I can see him giving information to Goldman under those circumstances.
At one time I was really into spy novels, and quite a few of them discussed how agents were recruited. Basically you get someone to give you a piece of trivial knowledge, then use that to blackmail them into the information you really want. I think this happened once Chief Delk's death resurrected Will Pope's career. Basically he had to continue to feed information to Goldman, or Goldman would reveal his source.
The rest of my speculation is based on spoilers:
We know that Gabriel and Pope won't be in Major Crimes. We know that Gabriel can be self-righteous, and we've seen Pope be irrational were his career is concerned. (Operation: Swift Justice anyone). What if Gabriel finds out that Pope is the mole, and confronts him with this knowledge? Pope's entire career has been leading to this moment, and someone he would see as a sanctimonious ass is threatening to ruin it. People have killed for less. He attempts to cover up the murder, Brenda uncovers the truth, and has to arrest her former mentor for the murder of her protege. After the year she's had I can see this convincing her it's time to leave the LAPD. (I may be biased by my perfectly reasonable dislike for William Pope).