According to ZDoom Wikia:
Zandronum is an open-source multiplayer source port forked off from the most recent development of the source port Skulltag. Created by the former Skulltag development team, they forked off into their own port due to conflicts with Skulltag's creator. Because it is based on Skulltag, Zandronum is also based on ZDoom and on the GZDoom renderer.
I found this a few days ago when I found Skulltag site was down for some reason. I found that there were conflicts (again) between the admin and developers, so the former developers wanted to create a new open source port (much more like (G)ZDoom) without having the new weapons, monsters and items (minigun, bfg10k, blood demon, etc). Post your thoughts here.