I thought one of the theories for the bronze age collapse was that the "sea peoples" were rampaging around like Vikings and pillaged the coasts so badly that trade fell a part? Though from what I've read/heard that collapse is one of the great historical mysteries.
From what I know, those sea peoples were very unlikely a forein power. It seems that it was just a cycle of "food shortage -> people go pillage -> more food shortage -> more pillage -> ..." with more and more local people having no other choice than becoming "sea peoples" to survive.
Additionnaly to the sea peoples, some possible causes that I know (some of them might be wrong or imprecise, I'm not an historian):
+ Some catastrophy in Europe forcing a lot of refuges to the mediteranean sea
+ Civilisations too reliant on trade (this is the Bronze Age, so most tool used bronze, but the major civilisations had very limited access to tin, needed for bronze, so trade was required)
+ Civilisations too centralised, so no part of the society was self-sustainable (even farmers didn't have the ressources to farm without the governmental help, and knowledge on how to farm efficiently without the scribes/priest/...).
+ Degradation of lands. This is the first time in human history that lands are over-exploited. So the production slowly decrease without reasons understandable at this time.
+ The writing system was so complex that literacy was not something you could learn quickly. So if people that can read are killed (eg, in a revolt because of food shortage), then nobody can read anymore. Same for the use of complex weapons like chariots, you need a lifetime of training.
Note that those flaw make those civilisations look like very fragile, which is wrong. They were advanced enough to solve almost any problem they could encounter, and I doubt that if only one problem occured at once, they would have fallen because of it. But once they started falling, the whole system collapsed (in ~50 years, from what we know).
(And the fact that most literacy disappeared at the fall make the exact reasons of the fall a great mystery)
I imagine we're all fans of history to some degree, otherwise we wouldn't be playing Civ
