This. In my experience, taillights take a beating when mounted on a rack, because shock and vibration gets transmitted directly from the rear axle up through the rigid metal rack supports directly to the light (chainstay mounted lights don't fare that well either IME). I've had lights from various manufacturers break off and get lost. But an even bigger problem is lights turning themselves off, probably due to momentary loss of contact in the battery compartment (soft switches are evil on a tail light).

Not really sure how to fix this, other than to mount the light on the seatpost, or even on the rider's helmet, where the shock is less.