Hi Eugene,
This is happening a lot lately.
Private drop catchers are registering everything that expires. Then after checking the traffic for a couple of days they get a refund for the domains that receive no traffic and the domains are released back for the public to register again. The grace period is something like 5 days after the initial registration.
The private drop catchers have to be in tight with an ICANN-accredited registrar to be able to do this. Because most registrars do not allow refunds for registrations and even fewer will allow for thousands of refunds per day.
This practice used to be considered a manipulation of the system, but it seems to be fair game nowadays.