“Bitch, I’m a bus” was a comment from a schoolbus driver that became an in-joke (you say it like “I’m a boss”), then a subreddit full of pics and videos. After a year, the subreddit is still pretty active (about two posts a day) for its size (5,889 subscribers).
I think there are two main reasons why “Bitch, I’m a bus” is funny: The obvious one, previously proven by “Deal With It” and found-face memes, is that anthropomorphizing inanimate objects is fun.
The second is some Poe-like phonologic: The phrase sounds like the first half of “Bitch, Imma bust yo head” or some other such urban-thug sentence. Your mouth gets the satisfaction of the bit-off Jesse-Pinkman-style “bitch”, the Samuel Jackson “ah”-ing of the “I”, the drawl of the “uh” and the ending hiss of the “ussss”.
So, yeah, good subreddit full of buses being badass.