On top of that, Philemon personally hated Sauros.
There is no reason not to act given the chance.
I guess that’s to be expected.
Whatever you say about Sauros, he was still a lord at that time.
Even if he lost his territory, he had the protection of the first prince's faction, he wouldn't have been able to lose his standing if the person responsible wasn't a noble with the same level of power as him.