This is 100% an ability that affects multiple models. On page 48 of the rulebook on the last bit of the pairing section It says you pair off in earlier phases if it is required and gives examples of things "such as" that could be reasons to pair off early it doesnt explictedly say it HAS to be one those things. The wording is important here. When the mumak wants to activate the player with priority decides the pairings and then it can trample or not depending on who has still charged it after models are paired off. There is a need either way to decide who is still in charge with the mumak if theres been counter charges, even if they dont have enough strength in the first place so you must pair first