[Decib0rt]

Forgetting to renew the domain, a common torrent admin problem. Hope the guy is OK.
The tracker is on or more subdomains like t1.the-zomb.com and that resolves to the same IP of some domain monetization service as the top level does.

However, torrent clients are janky and weird. It may be your client either shows the tracker online because it isn't querying DNS for a new IP or because the current IP doesn't return errors for invalid subdomains since everything is routed to the same placeholder page (so you won't get the 404/403 you would normally get). You may also be able to upload/download through existing p2p connections.