He wasn't perfectly convenient for US border guards when he was returning to Canada, and/or some guards were just in the mood to attack. Peter Watts has not yet been sentenced, so it could be anything from suspended sentence (but he might not be able to travel to the US again ever ) to two years in prison.
Contra preconceptions-- Watts is white. One of the violent guards is a woman.
From james_nicoll, just the facts:
Toronto author Peter Watts has been found guilty of assaulting, resisting and obstructing a U.S. Customs and Border Protection officer.
Jurors returned the verdict today in St. Clair County Circuit Judge James Adair’s courtroom. He faces up to two years in prison when sentenced April 26.
From coffeeem: Mostly about jury nullification, the inadequately publicized right of juries to acquit regardless of the law.
Jury nullification is one of those ambiguous things, but I believe it's valuable because it means the government will not be worse than the public.
Big roundup with context and explanations of why the police are getting away with too much.
At this point, the conviction has been announced way down in an open thread at Making Light. I expect that there will be extensive further discussion there  or at a new top level post.
 Of course, he might not want to.
 The thread is almost entirely jokes, starting with bar jokes and spreading into knock knock jokes, elephant jokes, etc.