I haven't done this in a long time. When I was studying in the UK, it happened a couple of times. I would start to write in my journal when suddenly the pen took off on its own. I struggled to keep hold of the pen. What was weird was that I could not control what I was writing and I used words that are not in my usual vocabulary. Best example: I used the term "whilst." With as many members as we have here, I thought it might be possible to find others.
This happens to me a lot! Sometimes I sit down to write an entry in my journal/diary or try to finally finish that book that I've been working on for 3 months and then my hand goes "F you!" and writes something else that I would never had come up with on my own! I thought this only happened to me.