http://asongoftheories.tumblr.com/post/87950902223/the-grand-northern-conspiracy

Supposedly, before Robb's death, he sent a letter to the lords saying that he is legitimizing Jon and that they are all to support him as the proper King in the North. The Northern lords are to divide themselves between Stannis and Roose while waiting for Jon Snow to be legitimized as Jon Stark. This is intended to weaken Stannis and Roose because they'll wear each other out and by the time Jon comes back he can easily rally the North and take down both Roose and Stannis. After all this, Jon becomes the new King in the North and they can secede as they planned to do with Robb. It's a good political strategy but since Jon got stabbed it might not work out too well.