Why did the Cabbie Punch me?

When I first started cycling in London around 30 years ago, many people warned me about the dangers of Black Cabs performing U-Turns without warning, typically as a cyclist was passing them in an otherwise stationary line of traffic. However I found Black Cab drivers to be amongst the most courteous of fellow road users, seeing themselves alongside cyclists as part of the solution to problems of mass car ownership.

The construction of segregated cycling infrastructure under Boris Johnson as mayor seemed to cause a sudden turnaround in the attitude of Black Cab drivers, varying from minor resentment over roadspace to calls (mainly on twitter) to threaten cyclists and a noticeable lack of willingness to give way or show much consideration.  I had generally put some of the twitter chat down to a small bunch of angry old-timers with too much time on their hands - people like me in fact - but with a black cab rather than a bicycle.

Then rather unexpectedly, whilst I was slowing down for a right turn, a black cab driver drew alongside me, leaned out of his window, and punched me, before driving off.

It was so unexpected that I didn't really have time to react. The punch wasn't hard, but it was enough to knock me off my bike and bruise myself against the kerb of the central reservation. Two passers-by immediately ran over to help; one called out the registration plate of the taxi to commit it to memory, but seconds later all 3 of us had forgotten it. I reported it to City of London Police as an assault rather than a traffic incident, and they found cctv footage of it, but not clear enough to identify the taxi, let alone the driver.

But the biggest mystery is why on earth did he punch me in the first place. There have been plenty of small incidents where right of way is disputed, resulting in a torrent of verbal abuse, and some incidents where it could be argued that Cab Drivers had driven in a deliberately threatening manner, but this wasn't one of those. We had had no previous contact, there was plenty of space on the road, it was quiet, nothing to provoke any action, let alone a punch. There was nothing threatening about my clothes - it was a woolly hat and plain jacket, no Brexit or GTFC slogans that might have triggered some-one with different views; no helmet or camera to suggest I was trying to catch someone out.

Your suggestions please - Why did the Cabbie Punch me?