More than likely, the particular game you're referring to has the default set so that the ADS isn't toggled, but there may be a option in the game to enable ADS toggling or something similar. Beyond that, it may just be the mouse malfunctioning. I had a Razer Naga stop working and the left click did what the right click was supposed to and the middle click and right click did nothing, so it's not too far fetched to imagine.
I would try looking in the settings first, though.
EDIT:
Just noticed that you'd mentioned that you are holding the button when it does this. In that case, the only thing I personally can imagine it being is the mouse is malfunctioning, but others on the forums may know better than I so I wouldn't trust me just yet.