Nor did he wonder why William acted so queerly.

You never wonder why a mule does things; you just fight it out and are satisfied if you win, and let it go at that.
Casey does not remember clearly the details of that night.

He knows that during the long hours William balked at a particularly steep climb, and that Casey was finally obliged to get off and lead the Way.

It established an unfortunate precedent, for William refused to let Casey on again, and Casey was too weak to mount in spite of William.

They compromised at last; that is, they both walked.
The light went out.