The only thing i can think of is that its down to poor balancing of the players skill at startup.
But then the forums are full of complaints from people that their parties get split up when entering servers.
I'm honestly not sure what you'd change to make sure that teams don't end up losing by massive amounts.
Enemy Territory had it right. Once everyone was loaded, there was a waiting time. Everybody made off on foot altogether. Not a waiting time once the server has loaded. Feels like everyone has an SSD at times.