I'm just hypothesizing here, but it could be a case of population pressure like that of the equally dramatic Migratory era during the late-Roman empire. There seemed to be a rapid uptake in warfare and fortification in places like the Italian peninsula as well in the period right before the BAC. My guess is the general European population might had reached a population level where there wasn't enough land to support people (with their current agricultural technology at least) or some widespread natural disaster (drought, flood) disturbed the balance of food production/consumption, so local tribes became much more competitive for land and resources, eventually leading to widespread intensive warfare. Those that were defeated then had to move away, either into the territories of the BA states, or forced other tribes to migrate and put pressure on them. While the militaries of the BA states might have handled the occasional intrusion by these "less sophisticated" groups, a constant torrent of desperate (and armed) people fleeing their homelands to gain refuge/loot eventually overwhelm the defenses of the BA states.