Sometimes BT maps are modified versions of maps from other gamemodes and games based on same engine. I don't know if author who did remake got permissions or told publicly that this is modified version of other map. It is just statement of fact and would be nice to find the real truth.
1. BT-(Ki)FieldWar-Lv - original map is AS-RealAssault (map was also ported to Tactical Ops game [TO-RealAssault])
2. BT-EaZyWaY - original map is TO-XMAS-TAZ (from Tactical Ops game) - it looks like map from AoT game was basement of BT version. The person who modified map didn't fix screenshot and other data inside map.
3. BT-Cliffs2 - original map is MH-Cliffs2 - person who modified map was too lazy to correct data inside map
4. BT-HotSexDisco - original map is CTF-HotSexDisco