The pressure tank has ruptured at the top on bulkhead weld. What will cause the over pressure?

Hard to say, other than the pump not shutting off. I would definitely replace the switch. This illustrates why well systems should have a pressure relief valve.

The pressure tank has ruptured at the top on bulkhead weld. What will cause the over pressure?

Are you talking about a pressure tank or a blladder tank?(see images) What does your pressure gage read when the pump shuts off? What are the settings on your control box? Let me know, Tom