Bracketing has to be set via a function button, so you have to be pressing one of the buttons to be initiating bracketing. Then you have to turn the main rear thumbwheel to how much EV you want. You would then get not just -1 but +1 as well....so it sounds more like to me that you just have Exposure compensation accidently set .
This is easy to do accidently, but bracketing cannot be done without you knowing
Have a look on the top of the camera where you push the shutter button - just to the right of that and slightly nearer to you than the shutter button, is another button with black & white triangles and a + or - sign. Switch camera on, hold that button down while turning the rear main thumb wheel. Look at the top small LCD readout and you will see the bracketing change. Turn it left or right until you see 0 EV
I reckon you have accidently been pressing that button, as well as the shutter button and you've turned the rear thumb wheel once or twice and accidently set -1 EV