Arthritis runs in both my mom and dads families. One of my uncles had those huge arthritis knots on his hands . It was SO bad. I have a knot that forms and goes away again over and over on my right thumb, so kinda know what I'm in for.
I think typing helps me to keep my hands limber as I do it whether it hurts or not... which is the key with arthritis. Once you stop using the arthritic joints they just lock up... not willing to let that happen.
I also injured my hands several times when I was young, so old joint injuries are often prone to arthritis. I've broken fingers, dislocated a thumb, etc while breaking horses.
So anyway that's what I blame this on. lol