Have you just mounted them, or mounted them and used the camera while riding the bike? The iPhone has sensor-shift image stabilization (what camera manufacturers are calling IBIS - In Body Image Stabilization) and if this is engaged (for example while filming) it would be much more prone to damage than if you are using the phone for, say, directions only with the system parked in transport mode.

At least this would be my assumption, I don't know where this problem comes from exactly.