I realize this is a little before history and therefore interpretive. I recall reading something about a shift that took place in Gaul, some time before the Romans came there. It may have been around 350 B.C. It involved a major tribe in central Gaul which was more or less dominant. Their capitol was sacred to the Celts, so much so that it was spared in major wars. I believe it may have been the capitol of druidry. Then there was a shift and another people, i think they were from the north, overtook them. I'm sure La Tene was involved in this somehow, where they the newcomers? the dispossed people may have formed a part of the celtic invasion of Italy. Maybe it was the Averni? Am I making this all up?
Added: The biturgies were influential in druidry, and Avaricum (Bourges) was the city that Vercingterox did not burn. I'm not talking about his era, but highlighting the importance of the city. Did the Biturgies displace an earlier people from Avaricum?