That may be, for lack of a better word, muscle memory/reflex for some people. I found out last semester, when I wrote out lecture notes on an ipad that there are many words I literally forgot how to spell. I can type them, and I can write them with a pen, but writing with a stylus is different enough that it took several seconds of concentration before I could write it down. I wouldn't be surprised if there are words or phrases at I always capitalize.