Never had that problem and hear first time of this.
Sounds like a broken keyboard button to me.
You should check that keyboard key with another game or software.
What about your system specs ?
Why don´t you give us basic info about your PC and its components ?
and why do you think a broken keyboard button would do this when i use the mouse to aim down sights?
Because you didnt specify which button you use to aim down the sight.
I for example use a keyboard button to aim / zoom in/out.
As i already said, i never had this happening on my PC and didn´t hear that happen on anyone that i play with.
PS : May be you should delete the files manually which are containing the keyboard/mouse settings and let steam repair the files
( steam will download them again ). That way you always can repair broken files.