I once heard that berserk is a kind of paranoia.
But considering berserk (and it's symptom) seems to be restricted in that area (I haven't heard of Japanese berserkers, or Chinese ones), I doubt it is a kind of paranoia. Rather, it was achieved by more artificial mean is reasonable.
But, also berserker looks and behave like a kind of extreme excitement and it seemed a certain family frequently had berserkers, generic facter is also possible. Like, mashrooms were more effective to a certain people.
By the way, some Japanese cook poisonous mashrooms.