Normally there is a 30 day grace period after a domain expires during which only the registered owners can renew. During this time frame the registrar/host will often change the dns settings to a page they have specifically setup for this often with the intention of profitting from the traffic. Hence you get adwords, search boxes etc.
Also I think a lot of domain purchases made through a third party/reseller are gobbled up by those sellers just before the grace window closes if they have good traffic value without ever hitting the open market again.