Some software incl Microsoft's
changes your default location when saving documemts/files.
It is trying to be helpful by remembering your last used location & changes the defualt location.
Happens in all versions of windows.
Never did get to the botton of which particular programs do this and when.
It is inconsistent and will appear to be random.
I tend to use 'Save as' to keep an eye on this and have also been surprised by a 'sudden' change of location