If you can't see the video, one of the gendarmes snuck around behind her while the rest kept her busy. He managed to tackle her, but she saw him, turned and started to raise the shotgun before he got to her and I'd say if he was 1/4 second later he would have got a chest full of shotgun. The gun went off when it hit the ground, not sure if the trigger got pulled or if it was impact that did it. Looks like a pretty old shotgun so I would believe it would go off from impact.
Oh, and if course as soon as she was tackled she started yelling for help like "omg help save me from them they are hurting me" unsurprisingly.