Awful lot of killing just because Robb Stark did not marry one of Lord Frey's daughters.
It wasn't just that, in the books Walder's oldest son (one of the only ones he actually liked) led the Frey forces that joined Robb and died battling the Lannisters for him. The one he intended to inherit the Twins (and one of the only diplomatic, intelligent Freys as well) was now dead.
On top of that, Robb was planning on leaving the Riverlands to go back into the North and wipe out the Ironman invasion meaning that the Twins would be a major target of the new Southern Alliance to get back to the North and there'd be no Northern defense from Robb.
Along with the disrespect of his Marriage Vow, all that combined is what set Frey off.