If only I had a rear view dashcam installed then at least I could try and decipher their morse code and semaphore at my leisure.
I've had people do that kind of thing before and often seems to boil down to 1 of 2 things:
-They magically think you can get out of their way and think you are the one holding everyone up and completely oblivious to all the traffic ahead of you.
-You've done something, or not done something, that has given them a bee in the bonnet so to speak.
On the last point I had someone do it awhile back because I didn't bother to indicate when pulling off a roundabout when they thought I should - they were following me as well so not like they'd have been inconvenienced by it anyhow. The roundabout is too small to indicate when going straight over without it potentially being more of a problem misleading other traffic than helpful so I don't indicate. The irony was after all their remonstrations about it they then were too busy on their phone to notice the lights changing at the next set of lights we stopped at...