I think it has a lot to do with entitlement, as well as growing population and wealth. There are simply more people around, who have more "investment" in whatever they do. It is easy to get into a "get out of my way" mood; because we rarely take a step back and realize that none of us are more important than the rest of us. And nowadays everybody is special and important, which doesn't help.
I've come to the conclusion that people are not ment to live in huge crowds. Our brains are wired for small, tightly-knight groups. Anything beyond that can only work with a solid "framework" - our social etiquette. But this etiquette does not adapt fast to changing conditions, like new modes of transportation. If there is any etiquette for any mode of transportation beyond walking, it is that the slower has to yield to the faster one. But here's the catch - nobody wants to slow down for somebody else, and there may simply be no room to get out of someones way. Add to that that bumping into somebody while walking is harmless, but ramming someone full steam ahead on a bike is a terryfying prospect.
The solution? No idea. Rules help to some degree; but they also prevent a social etiquette from forming. We do not question the relation of cars and people, because these two must not interact in everyday life - cars belong on roads, people on sidewalks. Whenever they meet, it has to be regulated (traffic lights, crosswalks, etc.).