People talk about the Alpha Legion and UM fights without mentioning the eternal grudges between the UM and Word Bearers over the ass-fucking they gave Ultramar during the Heresy? Or Ventris's murder-boner for Honsou? To quote Skinner: "You Ultramarines sure sound like a contentious people."
EDIT: If GW was smart, they could do this big throwdown where every Legion and warband the UM pissed off in the process of attaining their fuck-massive rolls of honor decides to team up and get revenge, because the only thing any of them can agree on is that the Ultramarines are a bunch of fun-ruining busybodies, and the UM are forced to call in pretty much every single favor they've earned because there's a 14th Black Crusade, and its headed right to their homeworlds.