Here are a few theories about what might cause Derek to say that line.
To buy the hospital. Maybe there's some bylaw that makes it so that to buy the hospital, the docs can’t also be Seattle Grace-Mercy West employees. If that's the case, we'd expect all of them to resign along with Derek.
As a threat. Perhaps, in a latch ditch effort to stop the hospital from being bought by the wrong people, the docs resign in an attempt to make Seattle Grace-Mercy West seems less prestigious and valuable (or at least delay things enough so that the lawsuit docs can sweep in and save the day).
Because he thinks he's the problem. We see Arizona (Jessica Capshaw) yelling at Derek in the promo, telling him "everything that you've done has actually made things worse!" Maybe, for some reason, he decides his resignation can help the situation.
Share your theories in the comments below!
Catch the next new episode of Grey’s Anatomy on Thursday, February 14, 2013, at 9 p.m. ET/PT onABC.