With the widespread adoption of Naue Type II bronze cutting/thrusting swords and organized infantry that rendered centralized chariot armies -- and the societies that depended on them for defense -- obsolete.
There was a book published not too long ago, by Eric Cline, titled "1177: The Year Civilization Collapsed", about the Late Bronze Age Collapse.
I initially liked this book. But I eventually came to the conclusion that it didn't really explain anything and concocted an overly elaborate framework that did more to obscure what happened than to clarify. It felt like that old joke about philosophy, the field where "one kicks up a lot of dust then complains one can't see anything." It ended up being so complex that it's not falsifiable.
I was much happier with the narrative in another book, "The End of the Bronze Age", by Robert Drews. Drews presents the military-technological explanation. It's simpler and it just seems to fit better. It also explains why social organization in the Iron Age was so different from the Bronze Age.