Eruptions of very large volcanoes can contribute to ozone depletion in a couple of ways.

Ozone molecules can be destroyed when they react with chlorine. Volcanoes can release hydrogen chloride into the air, and if they are big enough, it can reach the stratosphere, where the ozone is. If this happens, the chlorine from the hydrogen chloride can start destroying ozone molecules.

Another impact of volcanoes is from the tiny bits of ash particles that they release. The reactions that destroy ozone happen more easily on some sort of surface than in the air itself, and volcanic particles can provide that surface. Because of these effects, very large volcanic eruptions can result in ozone depletion for a few years afterwards. On the other hand, smaller volcanoes may not have any noticeable effect on the atmosphere, because rain and snow basically wash their gases and particles out of the air before they get to the stratosphere.