This might be a bug. I also had this happen a few times. If I edit a page with reviews enabled, every now and then the “Enable Reviews” box is unticked. I didn’t untick it, and it doesn’t happen every time I edit a page.
If I don’t notice that it’s unticked and click save, the reviews are disabled for the page and I need to fill in the details again.
I’m using Chrome for editing WordPress pages.