House Arryn, Stark, and Baratheon had a legitimate reason to rebel. Still, Hoster Tully had joined them because of a shaky betrothal and even managed to get both of his daughters married to other Lord Paramounts. The old trout had forgotten to whom he owed the original rise of his House, and there was no way in the Seven Hells that House Tully would get off as lightly as Stark and Arryn did. If Edmure and his newborn were still alive in Casterly Rock, that is.