I'm guessing it's more than one person, but anytime an "Add Serverdown yadda yadda yadda" named person joins a CS:S server, that server immediatly suffers a crash and restart. It was happening earlier today when I was playing on NG servers, and it just happened several times in a row now when I was playing on the IBIS Zombie Mod server.

So this is just a headsup, and if a server locks up, look for "add serverdown" in the console for people who recently joined.

Beware.