I'm not a finance guy..............

...............................but I would venture a guess that "human nature" is the unidentified culprit:

There’s rarely a good reason for a stock market downturn. I’ve been studying many of the historical bear markets and corrections that don’t get as much publicity as the usual suspects (looking beyond the Great Depression, 1973-74, Black Monday crash, the dot-com bust, Great Financial Crisis, etc.). The theme I’ve found in many of the other instances where markets fell is that very few have a catalyst for the downfall, even after the fact.

-Ben Carlson, as culled from here