Sorry to hear of your experience, but a little more reflection will uncover the cause. Many people drive around in a world of their own, they use it as an excuse to remove awareness from the world around them. You may have inadvertently angered this man and your lack of awareness has caused you to not see the whole event unfold. It is hard to know without all the details, If you are stationary in a car and someone behind you is stuck and loses patience and has now began blasting the horn, this is a situation that now needs your utmost attention, this is a red flag. For you to be aware one moment that a car behind you is blasting the horn and then the next thing you know you are being punched in the head, you have allowed your awareness to slip. We as humans are not as safe as we like to think when out in public. You need to keep your wits about you at all times and be prepared to defend or protect yourself. I am hyper vigilant, perhaps due to some PTSD, but truth be known, these are dangerous times we live in. You should have turned your attention to the man blasting his horn, appraised the situation, appeased him if you could, if not, protect yourself, ensure the windows are up, the doors locked and drive away if you have too.