Your setting changed because you clicked on a link that has the change imbedded in it. We've encountered this before. It's a quirk of the software (if the person creating the link uses that setting, for some reason it's included in the link). I think I know which post is the offender - if I can find it again.
I should've recognized it yesterday, when I first saw it. (It's only now that I remember "the fix" for it.)