holy smokes i never noticed this thread until now. i've probably been one of the worst (if not the worst) offenders when it comes to this particular habit, however in my case there's a bit of a backstory to it.
when i was a kid, i learnt to type on a second hand computer my old man was gifted by one of his colleagues - family couldn't afford anything better. the shift keys didn't always work properly on that old computer, and had to be rapped pretty hard to register a keypress. after a while this got frustrating, so i stopped bothering to hit shift unless it was necessary (eg. when i was writing something for a school assignment, or my code wouldn't work if certain chars weren't capitalised) and a bad habit was formed.
Years later when I was in my mid-late teens, we could afford a proper computer, so I did eventually manage to kick the habit. Several years ago however, as my portfolio grew to a level that I never thought I could accomplish as a kid growing up lower-middle class, I decided to bring the habit back, sort of as a weird way to stay mentally grounded and not get carried away with my growing affluence, by serving as a constant reminder of my humble beginnings.
Although it's not really a habit anymore as I can fully control it and do use capitals properly when I have to (eg. writing reports/documentation or communicating with external parties at work). I've only been deliberately skipping capitals when writing casually/informally (eg. emails to family, writing anonymously/pseudonymously such as on here etc) for that reason. But if people find it irritating (though no one's ever said anything about it to me before), I can go back to typing properly, no problem.