Question

How does ozone depletion occur?

Answer 1

By reaction of stratospheric ozone with certain compounds, faster than it can be replenished naturally.

Explanation:

Ozone is naturally produced from atmospheric oxygen by radiation in the upper atmosphere.
http://gen.uga.edu/documents/air/activities/Ozone%20Demonstration.pdf
and
https://www.nas.nasa.gov/About/Education/Ozone/

Three gases that make a significant contribution to stratospheric ozone depletion, CFC-11, CFC-12 and N2O, have been monitored by GMD since the mid-1970s. Since then, numerous additional CFC's, HCFC's, and other halogenated gases have been incorporated into the measurement program as the number of monitoring sites increased. Most of the gases that are responsible for depleting stratospheric ozone are anthropogenic, but some, such as CH3Br and CH3Cl, have natural contributions as well.
https://www.esrl.noaa.gov/gmd/about/ozone.html

http://www.ozonelayer.noaa.gov/science/o3depletion.htm

https://ozonewatch.gsfc.nasa.gov/

https://www.nas.nasa.gov/About/Education/Ozone/