The braking force causes the rim as a whole to push back on the hub, this force is carried via the spokes. This force will cause the leading spokes at the top of the wheel to wind up whille the trailing spokes unwind, moving the rim to the leading spoke side.

At the bottom of the wheel the reverse is happening, so it moves to the trailing side. Net result is that the angle of the wheel to the hub changes with braking force = very bad handling characteristics.