From memory one hypothesis was that tin had become an essential trade good that was required for making bronze, and therefore using bronze for many of the times’ high-level technological innovations, especially construction tools, weapons, and for ships.

However, tin is rare, and at the time, there were only a few disparate sources of tin. It’s suggested the middle east sourced most of its tin from China via the silk road, and Ancient Greeks were getting theirs from deep inland European sources (possibly near Hungary, Brittany in France, or Cornwall in England).

This was fine during settled and undisturbed times, as the very long, convoluted trade routes prospered and grew.

But they were very susceptible to disruption during unsettled times, and it wouldn’t have taken taken much to be disrupted by large movements of nomadic warring raiders or groups of peoples, or particularly terrible famines or natural disasters located across critical trade routes.

And as states and cities likely isolated themselves behind city walls to protect themselves from the strife of the time, this only would have decreased trade even more, and suddenly they would no longer have the ability to make the essential tools and weapons their societies had become reliant on, in the numbers required, right when those nations needed them most.

This would have been especially ruinous if those nomadic raiding tribes, or groups of unknown origin like the Sea Peoples, had access to iron technology, which required only one more easily sourced metal, iron. Pure copper weapons, due to lack of tin to make bronze, would have been fairly ineffective against iron or bronze equivalents.

It’s a hypothesis, and not “proven”, but I’d say it’s a fairly plausible explanation for what likely happened.