Thompson was inside the Oakland gang for about a year when that happened, the biker who beat him was from another chapter. Tensions were mounting from a variety of sources, they were possibly about to go to war with a black biker gang, and the cops and feds were really beginning to crack down on the Angels. So they had a lot of nomads drifting around, which are bikers who do the enforcing and float between chapters. They're not known for their diplomacy and Thompson was probably too big for his britches, thinking he was untouchable by way of protection from Sonny Barger.
One of their rules is when an Angel starts beating on someone, they all swarm him. They pretty much have to do it as a unified front or they'll be viewed as a weak link in the chain. So Hunter got his ass kicked. One of the guys who befriended him in the Oakland chapter intervened, and put a stop to it after a little violence had been dispensed, and showed him the door. He was told he probably shouldn't return and had just become persona non grata. He was pretty sick of the scene anyway, and had compiled enough research to finish his book.
In 1967, nobody had ever been inside the Angels before, unless they were patched members or their bitches. Thompson was really cutting edge for his time, it seems like a boring anecdote nowadays, but the facts known about the Angels were smaller than these paragraphs. And the hysteria was pretty high, they were making the national news. People were scared shitless of them.
His book is still relevant. You should read it. The guy was an absolute master of the written word.