Exactly. it is strange you know. If game autobans some1 it is for a reason.
Could be problems on player end, like their mods etc.
u4g were experimenting with models and stuff alot, maybe they were testing it on your server and game reacted with autoban. As you said, it happened few times only. My guess they moved all their work to losa, so autobans stopped.
Magna and terminator... Who knows, maybe they have some moded stuff there as well. There are nsk, winter models, stuff that comes with maps etc. If you do not know how to install them, you get your game messed up. Difference is - they could be playing with it. So they keep getting autobans. It wouldnt be a big surprise, they all try to make their game "easyer", since they think all others do the same.
Thats a wild guess, but it has some background.
If we think a bit further from all models stuff, connection problems could be the case too. Maybe their connections are interfering somehow. Too many KEY packets lost, too high or too low bandwith, blocking of server requests, even intentional lagging with programs. Could be any of it, since it is important to normal gameplay and can effect server state - they are removed.