Well that was fast.

Japanese financial markets have experienced a stunning rise over the past six months, as a new government led by Prime Minister Shinzo Abe introduced policies of aggressive Keynesian stimulus paired with equally aggressive monetary easing by the Bank of Japan. The surest, steadiest bet on global financial markets in that time has been on rising Japanese stocks and a falling yen that makes Japanese exporters more competitive.

Until Wednesday night, that is. The Japanese market tanked in epic fashion in Thursday's trading session, with the Nikkei index falling 7.3 percent. The yen appreciated 1.7 percent against the dollar. Here's the one-month stock chart of the Nikkei:


Here's the thing, though. There wasn't really any news overnight that would justify a swing of that magnitude. Analysts are blaming the drop on some weak Chinese economic data and hints from Fed Chairman Ben Bernanke on Wednesday that someday, eventually, the Fed will taper its own monetary easing. There is no way on earth, though, that those kinds of tidbits would justify the kind of moves that were evident in the Japanese market.

So it has all the markings of being driven by some weird mix of psychology and the details of the inner workings of markets, of a lot of bullish investors either choosing or being forced to unwind bullish bets at the same time. It also is a reflection of the risks that arise when markets are so overwhelmingly driven by policy, in this case monetary easing from both the Bank of Japan and Federal Reserve. As wild swings in the U.S. markets Wednesday in response to Bernanke's testimony show, any little blip from central banks can cause outsized moves in the markets.

That said, it's worth keeping the 7 percent drop in Japanese stocks Thursday in perspective. Over the longer horizon (this chart shows the last six months), "Abenomics" is still looking pretty great for Japanese companies and investors:

Still, Thursday's drop suggested the rally may be a bit more fragile than it had appeared.