As you can see from the URL Pink mentioned, I have had some experience
answering a question like this.
It may have to do with the magnetism in her body. Something like a
hormonal shift could possibly change that, and it could have affected
the watches from that point out.
However, there is not very much medical documentation on the subject.
It seems to be more anecdotal stories, and most Dr?s tell their
patients (as I?ve had a number of family members with this issue
myself, and have heard first hand) recommend putting a piece of
leather or something on the back of the watch, and that (usually)
stops (no pun intended) the issue.
Does she work in a position where she is exposed to a lot of
electricity or magnetism?
I would also recommend talking to your Dr and seeing if they could
test for higher magnetism or electricity levels in her body.
However, many people all over the world claim this happens, so your
mother is not alone. It just seems like no one knows exactly why yet.
If this answer requires further explanation, please request
clarification before rating it, and I will be happy to look into this
Google Answers Researcher