If you mean your shot happens without you releasing the mouse button then there a 2 things that it could be. If you are using a wireless mouse your batteries may be getting weak and need to be replaced or your receiver needs to be realigned. If it is a wired mouse then your contacts inside the mouse are becoming worn and you need a new mouse. Those are the only 2 things that could cause your shot to happen while you are still holding down the button.
If that is not what you mean then I have no clue.