I've always seen it as a way of peacocking. A way for people to make themselves stand out from others. But I think it also stems from a mindset of "I'm aware that professional communication involves proper capitalization, but I'm not going to bother because I don't feel the need to communicate professionally to the person I'm typing to"
I'm fine with it as long as everyone is typing in lowercase. But if the culture of a company or community is to type a certain way, there's some level of disrespect (or rebellion) by doing the opposite.