Fake clients.
The server spoofs clients to make it look like people are connected. These servers USUALLY eventually get delisted. There's a blacklist out there you can put in your blacklist file to block most of them.
It could also be redirecting you to another server right when you connect.
Here's some blacklist files:
http://tf2app.com/blacklist/lists.phpI recommend "The No-Nonsense List". Just download that, and drop it in team fortress 2\tf2\cfg\
(You want the "Official in-game blacklist version" if you pick a different one)