No it wouldn't be that as he said it happens when he loads a new singleplayer game, not when he starts the game itself. That parameter would cause gmod to connect on startup.
If I recall, Garry had changed gmod to, when making a single player server, start a listen server. This was due to some compatibility error with Toybox. My best suggestion that would work would be to rename your garrysmod folder (this is so steam does not recognize it, then re downloads the content) then put your addons back one by one until the problem happens again.