(like) a red rag to a bull an object, utterance, or act which is certain to provoke or anger someone.

☞ The colour red was traditionally supposed to provoke a bull, and is the colour of the cape used by matadors in bullfighting.

2013New Statesman There's no doubt that while the phrase 'tuition fees' is like a red rag to a student bull, a capped graduate tax is not.