I agree some GOPers are souring on him

but the Syria pullout is the reason. The reaction to his announcement includes a broad group replete with resignations and condemnations in unusually stark terms .

They don’t trust Trump to not initiate other troop removals. It’s Domino Theory  all over again. The slightest retreat from global military hegemony is akin to surrender. Regardless of the rhetoric otherwise, they are devoted to nation building and being policeman to the world. No cost is too great.