I'd have to say IME it's moving to the head of a stationary queue of traffic, including being able to use bus lanes in the rush hour. Some drivers really don't like that. (Got windscreen washer sprayed at me as I passed a van driver who was stuck in a queue the other day. He found it hilarious - laughing to his mate as I looked back. Nice.). Or maybe them being held up by a cyclist in primary position when they are desperate to overtake. I see that quite often. (I'm not suggesting all drivers get upset by cyclists, but it's interesting to think about which are the actions that seem to set the odd ones off!).