What causes people to have a fit?

It depends on what kind of "fit" it is. If it is a seizure the causes could be very different than someone getting emotionally upset and "pitching" or "throwing" a fit. There could be medical reasons for any fit, even an emotional one, but sometimes people react very strongly to something and get upset and start throwing things. And it depends on the person what could cause this. Finding out that a spouse is having an affair could trigger it in one person... and just missing a phone call could cause it in someone else.

Epileptic seizures can be caused by certain kinds of flashing lights in some people, and I have seen a seizure caused by a doctor putting on a cast too tightly... so there are various causes there as well.