If I understand the question correctly you're looking for something along the lines "Are there any momentous events or shifts in human history for which there are no apparent or agreed upon attributable causes?"

It's difficult to discuss the question without meandering a bit into the territory of "history's mysteries" and other such pop-history you might find on the History Channel but I can think of several events which might interest you and give you cause for further investigation of your own.

The fall of the Mycenaean civilization in the late Bronze Age is interesting from an archaeological perspective. Mycenae was a powerful player in the Aegean. They had a very strong military presence, vast trade networks and a thriving culture which absolutely dominated the region from the 17th to 15th centuries BCE; however, for reasons historians disagree about, their civilization fell in the 14th century. Their massive coastal fortifications were destroyed, their forts razed, cities burned, language and culture lost to time. This period is known as the Bronze Age Collapse and numerous notable civilizations fell. Some historians argue this was the result of natural disaster, others refer to the invasion of the Sea Peoples (which in itself is a rather interesting topic) but there is no single agreed upon cause for this period of turmoil which sent most of what we might consider "Western Civilization" into a dark age.

Mycenae and the Mycenaean Age by Mylonas

The End of the Bronze Age by Robert Drews