Partly it's because they are engaging with the world from a primarily emotionally framed context at all times, which results in a stream-of-consciousness style of speech and writing. Especially when they get really worked up about something and their self-control melts away more thoroughly.

It's also not just from traditional conservatives. I serve on the board of my housing community, so I see a ton of this kind of thing from the mostly liberal neighbors living near me in a primarily gay community in the USA. Conversations on our local community forum site, or maintenance requests submitted to us are frequently filled with angry ranting nonsense that is barely legible.

Sometimes someone will rant for a paragraph or two in ALL CAPS and insult everyone else living nearby, then their tone totally changes for the final paragraphs of the email. As if they became too angry to continue, stepped away for a bit, then came back later slightly calmer and continued writing with a more relaxed tone. Why they don't restart the email at that point, I have no idea, but that communication pattern is really quite fascinating and sometimes worrying.