Why Does a Cut Avocado Turn Brown Even in an Airtight Container?

I put half an avocado in an airtight container in the refrigerator, but the exposed surface still turned brown by the next day. Is the browning mainly caused by air that remains in the container, or does it continue for other reasons once the fruit has been cut?

I’m curious whether the storage method makes a meaningful difference, or whether some discoloration is simply hard to avoid.