It appears, from his report on the trial, that he was basically found guilty of assault for failing to immediately obey an order by a border guard to drop to the floor ... after being punched in the face. (He paused to ask "why".)
The problem here isn't the trial, or the prosecutor, or the jury: it's a law that's drawn so broadly that failing to immediately obey an order from a US border patrol guard is classified as "assault".
(If this is remotely typical of the breadth of the law, then most of us are guilty of serious criminal behaviour and should turn ourselves in. Or something.)