It was a frenetic day of trading action on Wall Street.  At one point, the price-weighted Dow Jones Industrial Average plunged 1,597 points, or 6.3%, which was its largest intraday point loss in history.  The selling wasn't as pronounced in the S&P 500 and Nasdaq Composite , but it was significant nonetheless as those two major averages dropped as much as 4.5% and 3.7%, respectively, at their worst levels of the day.

There was a closing rebound try that made things look better on a closing basis, but that's not saying things looked good at the close.  The Dow Jones Industrial Average ended the day down 4.6%, while the S&P 500 lost 4.1%, and the Nasdaq Composite fell 3.8%.

The reasonable question on everyone's mind is: why?

It is a question that won't have a truly satisfying answer, because there wasn't a clear-cut fundamental reason that was responsible for the scope of today's selling interest. 

The concern that the stock market is overvalued has some application to Monday's selling effort, yet the thrust of the sell-off was mostly technical, mechanical, and psychological in nature.

The price action got things going. 

There were some decent-sized losses at the start of trading on follow-through selling from Friday's broad-based retreat, yet the major indices all snapped back quickly. 

The Nasdaq Composite, for instance, rallied 111 points off its opening low.  That rebound effort, however, lost steam and when the Nasdaq's opening low was challenged later in the session, and failed to hold, sellers stepped up their activity.  The same dynamic played out for the other major indices.

The selling efforts then intensified when the S&P 500 broke through technical support at its 50-day simple moving average (2717).  The Dow Jones Industrial violated its 50-day simple moving average as well, which led to additional selling interest that got compounded as stop-loss orders got triggered and investor fear spiked on the rapidity of the selling activity.

The CBOE Volatility Index soared more than 100% today (38.80, +21.64, +126.1%).  The spike in the VIX Index, which is euphemistically referred to as the market's fear gauge, reflected a rush to protect portfolios against further losses in the near term.

There was a related flight-to-safety into the Treasury market, which had been little changed for a good part of the day before catching a healthy bid that saw the yield on the 10-year note drop 12 basis points to 2.73%.

The S&P 500, which had been up 7.5% for the year as recently as January 26, has now surrendered the entirety of that gain, losing most of that ground over the last two trading sessions.

Every sector finished deep in red figures.  The scope of the losses was reflected in the fact that the utilities sector (-1.7%) was today's relative strength leader. 

The financial sector (-5.0%) suffered the largest pullback, feeling the added weight of losses in Wells Fargo (WFC 58.16, -5.91, -9.2%), which received a supervisory order from the Federal Reserve saying the bank cannot grow any larger than its total asset size as of the end of 2017 until the Federal Reserve has concluded Wells Fargo has sufficiently improved its governance and controls.

The range of losses for the remaining sectors was 2.7% to 4.6%.  

As one might expect on a day like today, down volume swamped up volume at the NYSE by a roughly 12-to-1 margin.  Declining issues, meanwhile, swamped advancing issues by a nearly 9-to-1 margin; and trading volume overall was very heavy with 1.32 billion shares changing hands.

The lone economic release today was the ISM Non-Manufacturing PMI Report for January.  It was stronger than expected, checking in at 59.9 (Briefing.com consensus 56.7) versus an upwardly revised reading of 56.0 (from 55.9) for December.  The January reading was the highest reading for the index since August 2005.

The Trade Balance Report for December (Briefing.com consensus -$52.3 bln) highlights the economic calendar on Tuesday.

• Nasdaq Composite: +0.9% YTD
• S&P 500: -0.9% YTD
• Dow Jones Industrial Average: -1.4% YTD
• S&P Midcap 400 Index: -2.7% YTD
• Russell 2000: -2.9% YTD