Why do people yell at other drivers?

This morning I was driving down a wide, straight national speed limit road at the speed limit when an old man trying to turn out from a side road decided to yell “slow down” at me as I drove past!

Like, what? Why? In what possible world could me driving at the speed limit minding my own business warrant that? Even if I’d been going slower he still wouldn’t have had time to pull out in front of me safely, which is clearly what he was trying to do.

This was three hours ago and I still can’t figure out what the point of it was.