Could be due to your having a different printer installed than the person who wrote the document. It'll reformat for the different print metrics and want to save the result.
Going back a few versions, putting "page n of m" in the header or footer could do this - it counted how many pages there were in the document on opening it, replaced all the instances of "m" with the same number, and regarded this as a change that needed saving. I don't know whether or not they've now fixed this.