Not sure how it happened, and I didn't see what happened I'm only going from what's been said. There was a backdoor used to gain access to the server which sent people to random servers and caused graphical issues for users. I'll have the server offline redoing the entire server as when it was set up I had a lot of people working on it at once.
I still don't think there was a backdoor, but it's possible. Not all my addons were gmodstore addons. I used some websites that communities posted addons that weren't good enough to be in gmodstore or were rejected so instead they post it there.
Either way. Server will be down for a bit, pulling an all nighter to get this done. I don't mind it because I needed an excuse to get myself to redo the entire server.