Do you have a controller plugged in, abnormally? (joystick, wheel, etc)? I've found that having a joystick plugged in will cause my rifle to shoot uncontrollably and sometimes move around.
Yup,it's probably a joystick plugged in as Saghen mentioned. I had it happen to me last week after I re-installed the game. I didn't spin around but my gun wouldn't stop firing.