so called because red objects supposedly infuriate bulls
red rag in American English
noun
something that arouses anger; a provocation; red flag
A mere mention of the incident is a red rag to him
Word origin
[1690–1700]This word is first recorded in the period 1690–1700. Other words that entered Englishat around the same time include: additive, cocoon, lifeline, migrate, smash