What do you mean by avowed atheists? Just somebody who says they are atheist, or things like those atheist street preachers? I'm an atheist and I tend to get annoyed by street preachers of any kind. When I am going about my day, I don't want somebody to be trying to guilt me about my lack of religion or to guilt my family because of theirs.
I think intolerance comes in all groups, theistic or not. Personally, I have seen a bit more hatred from religious folk than I have from atheists, but that could be due to who I spend time with and my location on the map. While I've seen some very spiteful atheists, I've never seen any true hatred personally, only heard about it online. And usually, the people who do have spite are the ones who have been personally wronged by their religion (e.g. the LGBT).
The atheist hatred towards Christians would be america-centric. Think of the primary religion around here. Christianity. Even if you aren't Christian you are probably some sort of Abraham religion. Chances are, these atheists are ex-christians and chances are that their journey away was not a particularly smooth one. In most cases, they will lighten up in time after the wounds are healed, but I can tell you from experience, the journey away from the church is filled with an overwhelming feeling that you've been lied to and misled by Christianity your entire life. It is not fun.
I wish I knew a way to get rid of the annoying subway preachers. The ones who ambush you in nearly empty subway stations and just won't let go until you can run in a passing subway wagon away from them.
I don't meet them often (once or twice a year) but are they annoying. Nothing you can say seems to make them let you be because they have a mission to save you.
I'm thinking about trying to concert the next one to Satanism to see if makes her run.