Look, let me spell this out to you in simple words.
I hate it when pigeons sit on my roof and crap down my chimney; this is a thing that pigeons do. Sitting on a roof and crapping down my chimney is not a thing that Christians do. So, if I say, "I hate it when anyone sits on my roof and craps down my chimney, whether they're Christians, pigeons or insurance salesmen," then I'm casting unwarranted slurs on Christians by implying that there are some circumstances in which they will sit on my roof and crap down my chimney.
Now I can imagine you may have seen a few atheists get a bit pushy, but I suspect they were in a discussion that had already started, that people could choose to be or not to be in. I suspect they weren't parading around neighborhoods, knocking on doors and bothering people with no interest in discussing their beliefs. I suspect they weren't going on television and ranting about how anyone who didn't share their lack of belief was fundamentally immoral and not worthy of citizenship. I suspect they weren't dragging people out into the open and throwing stones at them or shooting them for their beliefs. These are not, generally speaking, things that atheists do. Getting a bit heated in an argument that's already started is a very different thing.
Dave