Yeah, as far as I can tell, this is caused when you try to join an almost full server, say 19/20. You hit connect, it starts loading, at the same time, 2-3 OTHER ppl also hit connect. Unlike T1/T2 it seems to allow all 4 of you to connect, but whoever actually finshes loading/connecting first "wins", the rest get "rejected by server".
This is just a guess based on how it was happeneing to alot of ppl last night. Impulse East was full about 2 minutes after it was brought up. Throughout the night Eq(owns impulse) said that the server info was listing 23-25 players connected, when the player limit is 20.
From all that, thats my best guess. Also, some ppl got this error when changing maps, apparently when it changes maps, the server appears to be empty, then a bunch of ppl try to connect, beating out someone who was already in the server