Fan motors have magnets in them. When the motor's stopped, the magnets essentially lock the motor in place until enough electromagnetic force is applied to start pushing them. Unless you're blowing really hard into the fans, air flow is generally not enough to overcome this.
The danger of spinning fans up is only when the SPEED of theat fan goes WAY too high and strains the bearings. There is no way that the air flow from another case fan can be so huge that it forces the fans of a video card to go that fast.