I have no idea. My first guess would be violent nutcase who had an axe to grind against the local constabulary. But, could be anything I suppose. Terrorisim, signal to Trump from the CIA, etc. THis is another event that I have no plan to stretch my imagination.
It seems to me ... that a local nut case would be unlikely to do or have access to sufficient timely intel to have placed the sheriff’s in the restaurant at that time—generally speaking.