It doesn't really have to be an external effect. Most ozone depletion is due to some other chemical reacting with the diatomic O2 in the upper atmosphere, preventing it from reforming into O3.
Ozone is continually created and destroyed up there, that's how it absorbs the UV radiation. O3 gets hit with an energetic UV ray, splits into O2 and monoatomic Oxygen, which then reforms into O3. The problem is if stuff gets up there that bonds with the diatomic O2 and turns it into something else, say CO2. That's how the majority of ozone is depleted.
So with your theory, perhaps a major volcanic eruption took place, which injected material into the atmosphere. Or a comet composed of a reactory material smashed into the planet.