Function problem

Whenever a movie clip envokes the following code it moves the whole stage, what gives?

function center (stageW, stageH) {
this.centerStageX = Math.floor(stageW/2);
this.centerStageY = Math.floor(stageH/2);
this.speedX = (this.centerStageX-this._x)/4;
this.speedY = (this.centerStageY-this._y)/4;
this._x += this.speedX;
this._y += this.speedY;
}

I would appriciate it if whoever fixes this could also explain why this is happening so I know for the future, thank you all for you help.