Paradoxically, perhaps, the fact that orthodox economics has a good deal to say about how the Great Depression happened itself suggests that there is after all something puzzling about the Great Depression. What's puzzling is not that the depression happened, given policies that were resorted to, but that such destructive policies secured wide support despite their often readily-predictable, adverse consequences. But to call even such perversity a "mystery" is to be guilty of hyperbole. After all, politicians are rewarded for appearing to "do something," and not for their command of "abstract" theories.
h/t: Cafe Hayek