Buying a domain name does not entitle you to that domain forever. In fact, you don't actually own it. If you register domain names, you must renew their registration, similar to how you would lease an apartment or a car. If a domain does not get renewed before the expiry date, then it will go back into the wild where it then has a chance to be snatched up by someone else.
Once a domain name expires, it goes through many stages before being released to the open market. Below is a timeline based on .com domain name; Days after expiration and Action
Day 1
The domain name registrar company makes the first of three attempts to contact the registered owner to renew the domain name. If the billing fails, or the customer does not provide a response, the domain name expires and the domain is suspended and all email, webmail or websites will not function. The customer can still renew the domain for up to 5 days without penalty.
Day 5
The domain name can be renewed by the registrant for the cost of a one-year renewal plus an $80 redemption fee.
Day 10
They add the domain name to an expired domain name auction.
Day 30
The expired domain name auction ends. If there are no back-orders and no bidders in the expired domain name auction, They list the domain name in a closeout auction. The domain is then released into the wild for someone else to purchase.