From ME!:
Pub brawl
Every political campaign should start off with a pub brawl, and the 1885 election was one of them. An overly loud comment had been picked up on, challenged by the other side, and resulted in a fighting, brawling, writhing mass of people.
Holmes happened to have been caught in the pub that particular night, looking for information, and was caught in the mass. His considerable skill at boxing served him well, and he made it out of the crowd in one piece.
"And Watson wonders why I pay no attention to politics," he muttered to himself.