User blog comment:Etindel/That did NOT just happen!/@comment-96.57.215.124-20130603153134/@comment-6735528-20130603165510

It was very obviously a collaboration between Walder Frey and Roose Bolton and the Lannisters. In A Storm of Swords Tywin tells Tyrion that he had secretly communicated with the instigators of the attack to secure an alliance and eliminate Robb Stark.