Well, that makes two of us!
For me, it usually happens when I'm reading something boring or uninteresting. Maybe even something that I don't completely understand or care about. That could be textbooks, old literature or just garbage.
When I read something I like and I'm invested in the story and the characters, I rarely think about anything else.
So if your mind suddenly becomes busy while reading, then perhaps that book isn't meant for you? If you'd be passionate and interested in it, I highly doubt your thoughts would be racing as much.
Another theory is that you simply have a lot on your mind at the moment and you could, for example, write some of that stuff up? Then try reading again. If it still doesn't work then let your mind rest. Perhaps it isn't the right time to read. Clear your mind. Try when more focused.
Good luck, hope this helped!