The ground cannot cause a fumble.
That’s an oft-spoken phrase in football. Actually the ground can cause a fumble under NCAA and NFHS rules even though there is no requirement for a runner to be down by contact. It would, however, be a very rare occurrence.
The veracity of that phrase lies in the fact that, 99.9 percent of the time, when the ball is freed from the runner’s grip as it hits the ground, the ball is already dead. It is dead because a part of the runner’s body other than a hand or foot had touched the ground before the ball touched the ground. That body part might be a knee, the side of a thigh or the forearm. Contact with the ground by any of those body parts causes the ball to become dead. Forward progress is marked at the foremost point of the ball when the contact with the ground occurred.
So how can the ground cause a fumble? While in a runner’s possession, the ball contacts the ground before any part of the runner’s body other than a hand or foot, and that contact causes the runner to lose control of the ball, then indeed the ground has “caused” a fumble. As you can imagine it would be a most unusual play. The runner would have to either stumble and try to use the ball to regain his balance, he could “lay out” or be flipped heels over head, so that the ball contacts the ground before the rest of the runner’s body, other than perhaps, the free hand.