For a long time I've believed the collapse happened because some people figured out how to make steel and went nuts over expanding then when their steel production ran out they lost their new territory. There's been plenty of discoveries of high carbon iron in the time period. Be it from meteors or just early attempts at heating iron with charcoal. Remember too that the metal ages mostly only refer to Europe and the time periods fall apart once you start looking at China and Egypt. After all King Tut had an iron dagger in the 14th century BC.