In the past, American politicians have been shot because:

1. The shooter was crazy (Reagan)
2. The shooter disagreed with the politics of the politician (Lincoln, JFK)
3. The shooter had failed to get a government job (Garfield)
4. The shooter had been paid off (Actually, Washington was never shot, but a member of his bodyguard had been paid by the British. They just found out before he had a chance.)
5. It was a personal grudge (Harvey Milk)

It could also be personal, but I doubt it.

Until we know more, any and all of these are possibilities.