I think it's become a bit of a cliche/clique'y thing amongst a certain population. I don't know its origins (tumblr emo crowd??) but I first encountered it in Silicon Valley. The Collison brothers used to love doing it, as did Altman. I feel it projects a kind of stream-of-thought with an aloofness, like "i dont care enough for correct form. language bends to my unique thoughts. read this if you like, i dont care lol".

All-lowercase comes accross as the text equivalent of a hoodie and jeans: comfortable, a bit defensive against being seen as trying too hard, and now so common it barely reads as rebellion.