alt Hacker NewsI guess it's a way to convey informality and rejection of "the norm" sacrificing legibility.
It's probably a way to express freedom, like the freedom of just refusing to consume unreadable walls of text that disrespect the reader. Freedom goes both ways.
not OP, but it's much simpler for me: when i edited/reordered a sentence, i hated when i had to fix the capitalization. and it's also faster to type without messing with the shift key.
eventually it became a habit and started to leak into more and more of my writing, including public writing. if i wrote something like a blog post, i would still capitalize it, though.