Haha! this used to happen to me all the time. I could read several pages before realising I was thinking about something completely different 😂. Often it’s because something has triggered my imagination or curiosity but there’s also a phenomenon where if you don’t understand the meaning of a word or phrase, your mind disconnects from the flow and your eyes keep going or rotating around that paragraph. I somehow trained myself to watch out for when these things happen, and I do it less these days