If I had to take a guess, it’s about perceived tone. Connotation becomes extremely important in online discourse.
Rather than “No. Here is the rest of what I have to say,” I recommend “Nah, here is the rest of what I have to say.”
It’s a lot less forceful, more conversational.

Why not? You didn’t know anyone in college who liked to speak differently sometimes, whether for fun, to be ironic, or to be flowery and “romantic”? People are figuring out who they are in college, some people do stuff other people might consider weird. That type of language doesn’t shock me at all.