[nwah1]

This is likely part of it. Particularly for pedestrians. But for cyclists, I don't think that explains it.

It boils down to the fact that cyclists are unusual in most places in the US, and people are impatient and don't like dealing with things that are out of the ordinary.

But there's also just something dehumanizing about driving cars, which causes aggression. Road rage against other drivers is nearly as big of an issue as the issue of hatred towards cyclists.

Of course, even pedestrians get angry at slow walkers, but when they do so they aren't in command of multiple tons of metal that can move at high velocities.