See the previous post on hitting, here, for some introductory material on where the data comes from and see the previous post on starting pitching, here, that covers what pRAA is.
NB: The Mariners' pen actually ended the season at -35.2 pRAA. The final series was erroneously not included in the dataset.
The bullpen actually held its own until June rolled around. Then came the nosedive. That's right when this happened by the way. Jesus Colome and Kanekoa Texeira had yet to be really bad for the Mariners when they were dumped for Sean White and Garrett Olson, who together combined for 13 runs below league average which is almost half of the total damage done to the bullpen between 1 June and 5 October.
The biggest collapse came when the pen managed to drop an astounding seven runs during one series against the Royals which was a cute series because Erik Bedard was right about to come back and Cliff Lee was still on the team. The Mariners were swept though. That can happen when Brandon League blows two saves in spectacular fashion.
After that it was a slow but steady downward crawl befitting a team with a below average bullpen. The Mariners relief corps peaked during the first series of the season and then managed to nearly bottom out in the league like their hitting brethren. They were saved from complete ignominy by the hilariously inept bullpen of the Arizona Diamondbacks who are way more interesting. If you're already going to be really bad, at least be historically bad.