A century of servitude to House Lannister usually left its marks on a family and House Tully was no different. Thomas had heard that Tyrith was holed up in the Keep somewhere and, at his father's behest, he decided to pay the lord of Lannister a long overdue visit from former vassals. The Riverlands usually kept on good terms with the Westerlands, and for good reason. See what happened last time?
"I see my lord of Lannister is somewhat engaged. Perhaps my visit should wait until you're less occupied, ser?"