Just bad manners,
When i cycle with friends, we'll cycle as a group, and split down into a line if traffic is heavy or we're holding people up...
I drive a car too so see it from both sides... but manners is all it boils down to...
This is exactly the issue, same as with neighbours who let their dogs bark incessantly etc. Maybe a eugenics program needs introducing then we`ll all be left with nice and polite people like us.