Combination of things, if I'm cold (literally and figuratively) or I'm still warming up, if I'm really pushing hard, or when I've been going too long and the fatigue sets in. I don't really know how common it is among other typists though, I've found I've gotten sloppier over the years due to auto-correct and google search 'guessing' being pretty damn fantastic.
Another story, when I was middle school, I took what I thought was a computer programming class that turned out to be a god awful typing class. Of maybe 30 kids, over half would completely fail to complete the assignment during the hour. The students that got through it were almost all my video game/computer geek friends. I personally hated the class as I thought it was a complete waste of time as I could already type fairly well. The carrot on the stick was that you could surf the internet, play games, whatever, if you completed the the assignment. Considering that the internet was wildly faster than my dial up line at home, I would burn through the class as fast as I could, literally always finishing before everyone else, spending 15 minutes tops on it.
Well, one day midway through the semester I overheard another student asking the teacher how I was always done so fast. This guy was a completely smug hard ass and was apparently a 'computer guy' prior to his teaching career. He had the gual to say it was only because I would make so many mistakes and that I was such a sloppy typists that I wouldn't be worth much to anyone later on. During the next class I made a point to hit 100% accuracy, declaring loudly to him so that the entire class would hear. I did that the entire rest of the semester, I loved it, he never showed his smugness towards me after that. Still really bugged me though, I realized even back then that computers were getting _good_ and handling typos, and that highly accurate typing was mainly for suckers.
As an aside, this was all during the era of the 'Ask Jeves's' of the world, I switched to google simply because it was so damn good at understanding the intent behind my typos. Incredibly, they're _still_ the only auto-correct I've used that understands and corrects the drifts!