Summer may not officially be over yet. But this was no sleepy summer Friday for the stock market.

The Dow fell nearly 400 points -- the biggest drop since late June. All 10 sectors in the S&P 500 were in the red. Oil prices plummeted. Gold was down. And U.S. bonds sank as well, pushing their yields higher.

In other words, there was nowhere to hide. TGIF? Not this particular F.

So what's going on? The good news is that there wasn't any really bad news. No major company reported awful earnings. There was no significant piece of economic data to spook people and spark fears of another recession.

In fact, the market has been eerily calm for most of the summer. And that might be the big problem.

Investors quickly shook off fears about the impact of the U.K.'s Brexit vote in late June. It was not, Wall Street decided, going to turn out to be the 2016 equivalent of Lehman Brothers collapsing -- a cataclysmic event for the global markets and economy.

Complacency may have set in as a result. CNNMoney's Fear & Greed Index, which measures seven indicators of market sentiment, had been in Greed or Extreme Greed territory for much of the summer. On Friday, it fell to Neutral levels.

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Sure, the U.S. economy may not be in fantastic shape. Auto sales have slumped lately. Many retailers have reported less-than-stellar-results too.

But America's economy is better than Europe, Japan and China. And many blue chip stocks have rallied this summer -- even though corporate earnings remain lackluster at best -- because the U.S. still seemed like the best game in town.

And perversely enough, investors seemed to feel that this was the perfect scenario for stocks. The Federal Reserve would be able to hold rates steady. The European Central Bank could keep buying bonds.

That may not be the case though. ECB president Mario Draghi hinted Thursday that there wouldn't be a heck of a lot more in the way of stimulus from bankers in Brussels. And several Fed members have been talking more about a need to soon raise rates.

The Fed's next policy meeting wraps up on September 21. Few experts think a rate hike is coming then. And there's little chance that the Fed will lift rates in November -- just days before the U.S. presidential election. Fed chief Janet Yellen is not insane.

But investors have been hooked on Fed stimulus for years. And the days of near 0% rates may now be coming to an end.

The rise in Treasury yields seems to suggest that the bond market is taking the Fed's rate hike talk seriously. The U.S. dollar has strengthened as well lately, which is what should happen if the Fed is raising rates.

And a stronger dollar could be bad for the market because it will make it tougher for big U.S. companies to compete overseas. That would hurt profits.

So Friday's market selloff may be a sign that investors still aren't ready to say goodbye to cheap money. But the Fed has to pull off the Band-Aid eventually. Here's hoping it's a swift yank as opposed to a slow, painful rip.