It's not just that it's the 'theory' that's all but confirmed in the books that Wayman Manderlay kills three Freys that are staying in White Harbor on the way to Winterfell for Ramsay and Jeyene Poole who is pretending to be Arya. This isafter giving them guest gifts, effectivy ending their protection of guest right.

They go 'missing' on the way to Winterfell and are never seen again. When asked what happened, Manderlay says that his host was moving slowly because they were carrying so much food for the Wedding. He says the Freys grow impatient and go ahead and he has no idea what happened, that it might have been the Brotherhood without Banners or some other bandits.

At the wedding he personally serves the Freys a slice of three separate pies (and eats two slices himself!). He goes on to tell his hosts to savor each bite, for he knows he shall.

He did all this with a gleeful smile on his face and then precedes to get very drunk and later asks the singer to play 'The song of the Rat Cook'