I believe it started on Reddit and came from meme culture (meme as in image macros, not Dawkins' definition of it). In around 2010-2012, there were tons of atheist memes and pictures on the internet which poked holes in religious arguments and pointed out various inconsistencies in religious belief systems. Religious folks needed some way to counter all of these anti-religious memes and pictures, and hence the stereotype of atheists as fedora-wearing neckbeards living in their parents' basement and subsisting off of Mountain Dew and Doritos was born.
You also mentioned that there's this stereotype of atheists shoving their views down other people's throats. I know not all atheists do that, but a lot of them do. Historically, this is what happened in Fascist Italy and the Soviet Union. A lot of modern atheists do it too. However, what I've found is that progressive atheists (e.g. atheism+) often try to impose their beliefs on other people, but most other atheists keep their beliefs to themselves, and rarely talk about them unless they're asked about it. At least half of my atheist friends are like this.
Personally, I'm not even slightly offended or annoyed by the whole fedora thing. The whole thing is so ridiculous it's hard for me to imagine how anyone could be offended by it.