Everyone left at the same time: your connection crashed.
World records: bullsh*t. Only way to get one is play races before the modders get to them. I just got one this way.
The thing I don't get about when your connection messes up and everybody else leaves the session is that the session actually continues totally smoothly apart from everyone leaving.
If you lose connection then shouldn't it boot you out and force you to load up a new session? Rather than tell you everybody else has left and you just carry on in the same session? I've never understood that but I'm not very knowledgeable about these things.