Pretty much standard for all timpanists college up to professional to sit. Facilitates tuning changes as well as brings the player down closer to the drum. Also not as much sitting as balancing on a stool. When I play I sit on a stool with my feet resting on the pedals.

Side note, most European orchestral percussionists sit and play everything, exception being when it would prevent playing the music accurately ie keyboard parts that require a large range.