He had done no harm to Brinkman or anybody else.

Why shouldn't he let out, and stand up for himself?
So, to Brinkman's utter amazement, he was met by a blow and a defiant challenge to "come on." What Brinkman might have done is doubtful, but at that moment Yorke and Ranger strolled by.
"Hullo! What's this?
A fight ?" said the captain.
"Rather," said Corder, now thoroughly strung up to the point.

"I say, Yorke, will you stop and see fair play ?" The captain hesitated a moment.

Any other fight he would have felt it his duty to stop.

This fight seemed to be an exception.