Don't think that guy getting a couple of fingers on his side cause him to go down, but I'm not going to cry about it. The calls all end up going both ways, it just depends on your perspective. (Husband is an umpire, so I hear/see it everyday.)
The point is, the reason it would be a fumble is that the ground cannot cause the fumble, but when the runner is not touched, it would have been called a fumble, even though he was down and the ground caused the fumble. Had the P'burgh player not touched him, it would've been Pittburgh's ball, even if Hasselback was down.