It starts right from the top, just like the similar situation in the Rails community.
When your founder and leader behaves a certain way, it gives everyone in the community license to act like that.
Edit: I seem to have touched a nerve here. My apology to anyone my comment offended.
Nonetheless, I stand by the idea that the culture of a company or open source is heavily influenced by the behavior of those at the top. How could it not be?
As an example of the other end of the spectrum, when I worked at Adobe it was a remarkably courteous place where people treated each other with respect even when they disagreed. I really appreciated that, and I think a good part of it came directly from Adobe founders John Warnock and Chuck Geschke.