I am one of those people, despite having written 6 books. I have a theory on it too. Something similar happened to me regarding choosing between unleaded green and leaded red petrol. I had a car that took leaded petrol. I changed to a car that took unleaded. Then I got a car that took leaded petrol again. So, on that last car, I was getting confused at the pump! Which does this car take again? I see it as neural pathways getting a bit mixed up.
Here is my analogy...
Imagine you are walking a route in long grass. If you keep walking along that route, you trample down that grass with each successive trip. Then imagine that you have to take a fork in that grass for a while, carving out a new route. At some stage, it is just as easy for you to walk down path A than it is path B. The same happens in a neural sense. You want to write "it is", but use the abbreviation instead. So you say "its", or is it "it's"? You get a kerfuffle in the brain, just like I had earlier even thinking about it! I imagine in my neural wiring, the fork in the grass has actually become more like a crop circle! I've been thrashing around in all sorts of directions over the years, unsure of which path to take that I've ruined the farmers crop and cause all sorts of damage. And because I still didn't know which "it" I should use, "it" continued. I eventually have drilled myself into a well of confusion out of which I will never emerge.