Well, didn't (& won't) watch the video, but for the question "Why do atheists preach"? (The video maker forgot the question mark, btw.) there is an easy answer. Human beings are biologically predisposed for (religious) belief. So, most people, even if they don't believe in any particular organized religion, need something to believe. Therefore they make something else into their "religion". Can be atheism, communism or any other ideology.