I tremblingly considered a moment, and then the devil of temptation provoked me to say:—

‘Well—to—to—be entertaining, I thought.’

This was a red rag to the bull. He raged and stormed so (he was crossing the river at the time) that I judge it made him blind, because he ran over the steering-oar of a trading-scow.

What does the expression “a red rag to the bull” suggest?