When a Dropper or Dispenser block is powered, it becomes sensitive to redstone updates and will dispense an item if an update occurs nearby (e.g. unpowered redstone being placed next to it, or the redstone powering it changes power levels). In this way, a powered Dropper/Dispenser can be made to fire repeatedly by placing an unconnected clock nearby (tested with unconnected redstone 1 block away).

I suspect this, when combined with the (now fixed) Comparator bug that cause it to constantly provide redstone updates, is why a powered Comparator pointing at a Dispenser or Dropper could cause it to rapidly fire.