[treprinum]

People in the past were underfed with all kinds of nutritional deficiencies. We can even today observe what a lack of some vitamins does to people in poor countries, e.g. schizophrenia from the lack of B3 or beri-beri from the lack of B1, scurvy from the lack of C etc. It's quite possible there was some sort of deficiency leading to neural damage manifesting itself as "dancing".

[User23]

Also there’s the ever popular ergot/mushroom conjectures for such phenomena. I think it’s a bit overused but probably partly true part of the time.

[ndsipa_pomu]

If it was a nutritional deficiency, then surely the symptoms would appear in different people at different times, but this event appears to be synchronised. To me that sounds more like some kind of neurotoxin that they were exposed to.