The sibling comment to yours mentions that this is pretty common on Twitter, and I'd guess that it started as a way to making firing off tweets from a phone easier (since the extra effort to hit shift when typing on a phone keyboard is a bit higher, and the additional effort to go back and fix any typos that happen due to trying to capitalize things is also higher compared to using a traditional keyboard). Once enough people were doing it there, the style probably became recognizable and evoked a certain "vibe" that people wanted to replicate elsewhere, including in places where the original context of "hitting the shift key is more work than it's worth" doesn't hold as well.