Before epilepsy was understood to be a neurological condition, people believed it was caused by the moon, or by phlegm in the brain. They condemned seizures as evidence of witchcraft or demonic possession, and killed or castrated sufferers to prevent them from passing tainted blood to a new generation.
I think it’s mostly because everything about us boils down to chemical reactions and we have no way of controlling them. Our brains do stuff before we can realise it or without realising it at all.