The issue of the disappearance of Rhaegar, Symond, and Ser Jared Frey causes increasing hostility between their former hosts at White Harbor, the Manderlys, and the Freys that follow Roose Bolton into the North. The three Freys served as envoys to White Harbor, arranging marriages and ensuring the loyalty of the Manderlys while Lord Manderly attempted to recover his surviving son and heir, Ser Wylis, who had been a hostage after having been captured at the crossing of the Trident. Once this was proved by his various agreements and his apparent execution of Ser Davos Seaworth, arrangements were made for Ser Wylis’s return.
What follows next is key: while feasting the returned heir to White Harbor, Lord Manderly secretly met with Ser Davos (whom he had hidden in the Wolf’s Den) and revealed that he was willing to follow Stannis provided Ser Davos did him a service by bringing Rickon Stark to safety from his present location (V: 393). Lord Wyman intended to travel by barge and litter to Winterfell, as commanded by Lord Bolton… but the three Freys were to receive horses from him so that they could travel more swiftly. Wyman then asks Davos if they have the custom of the “guest gift” in the south, and Davos says yes: you give them to your guests on the day they depart (V: 392). The salient point is this: guest right is sacred in the North, and it protected the Freys… but when Manderly gave them his guest gifts, they were no longer his guests and no longer protected.
What happens after that requires us to look at the wedding feast for Ramsay Bolton at Winterfell. Lord Manderly presents three huge pork pies and eats from them with gusto (taking two helpings from each, in part to prove they are not poisoned). So reassured, the rest of the wedding party eats, including the attendant Freys (V: 493). Notably jovial through the proceedings, towards the end the drunk Lord Manderly calls for a song about the Rat Cook (V: 497). All together? The three pies contained the three Freys, which Manderly wilfully (and joyfully) fed to the unsuspecting guests at the feast as a means of having his revenge. The song about the Rat Cook indirectly reveals this, as the tale concerns how the Rat Cook served the Andal king his “prince-and-bacon pie”.