What is the main reason for the depletion of the ozone layer?

Chemicals such as chlorofluorocarbons (CFCs) and others destroy the ozone layer. These chemicals are found in all sorts of things (see here). For example, Carbon tetrachloride (CCl4) was once a popular cleaning agent but is now banned.

The image below shows a simplified example of how a CFC molecule impacts the ozone. CFCs do not directly destroy the ozone layer. There are many chemical reactions involved in this process. CFCs decompose into other chemicals, eventually producing chlorine and chlorine monoxide molecules that do directly destroy the ozone. Learn more here.

In the case of ozone depletion potentials, the reference gas is CFC-11 (in formula #CFCl_3#). CFCs (such as #CF_2Cl_2# and #C_3H_3Cl_3#) and HCFCs (such as #C_2F_3HCl_2# and #C_2FH_3Cl_2#) are ozone depleting substances. But the most important ozone depleting substance is Halon-1301 (#CF_3Br#) since its ozone depletion potential relative to CFC-11 is 12.

Methyl chloroform and methyl bromide are other ozone depleting chemicals but their ozone depletion potential relative to CFC-11 is 0.1 and 0.4 respectively.

These gases all together cause ozone depletion in our atmosphere.

See here for more ozone depleting substances.

You may also be interested in learning more about basic ozone science from the Environmental Protection Agency in the US.