Image source: Getty Images.

Stocks posted a mixed start to the trading week, with the Dow Jones Industrial Average (DJINDICES:^DJI) rising slightly as the broader S&P 500 (SNPINDEX:^GSPC) index ticked lower.

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Data source: Yahoo! Finance.

Oil prices spiked after OPEC member nations agreed over the weekend to reduce production. That boost generated a 2% uptick for the United States Oil ETF (NYSEMKT:USO). Similarly, the Direxion Daily Gold Miners Bull 3X ETF (NYSEMKT:NUGT) was one of the market's most popular exchange-traded funds following a rise in prices for the precious metal.

As for individual stocks, Chipotle (NYSE:CMG) and Alexion Pharmaceuticals (NASDAQ:ALXN) stood out with big moves on Monday.

Chipotle's new direction

Chipotle shares gained over 3% on heavy trading volume after the company announced a shakeup in its top management ranks. Co-CEO Marty Moran will be stepping down from his position as the restaurant chain's founder, Steve Ells, becomes its sole CEO. 

Image source: Chipotle.

The move comes shortly after Chipotle revealed widespread problems with customer service that are frustrating its efforts to recover sales growth momentum following last year's bacteria outbreak. More than half of its locations, executives said in a recent investor presentation, have received a grade of "C" or below in the company's internal customer-service tracking.

Ells is hoping to make major changes to the chain's operating structure to raise those scores back up to industry-leading levels. "Operations have become over-complicated," he said in a press release, continuing:

I'm very much looking forward to relentlessly focusing on ensuring an excellent guest experience, removing unnecessary complexity from our operations, championing innovation, and pursuing our mission of making better food accessible to more people.

Chipotle had predicted it would bounce back to sales growth in 2017 following a year of massive customer traffic declines. That plan has been complicated by the new challenges of poor customer service and elevated employee turnover. This CEO shakeup indicates that the company is taking the threat seriously and will be rolling out fundamental changes aimed at stemming the customer traffic slide.

Alexion Pharmaceuticals' management changes

Alexion Pharmaceuticals shares slumped by 14% following a surprise announcement that CEO David Hallal has resigned, effective immediately. In a press release explaining the change, Alexion said only that Hallal was leaving for "personal reasons," and that board member David Brennan will serve as interim CEO while the company searches for a permanent leader. Additionally, Chief Financial Officer Vikas Sinha has left the company "to pursue other opportunities," and his post will be filled by David Anderson.

Image source: Getty Images.

The biotech noted that an internal audit investigating the marketing of its Soliris drug -- which has caused the company to delay filing its third-quarter results with the SEC -- hasn't yet uncovered any issues that would require a restatement of its previously reported results. That provided little comfort for investors, though, who fear that this management move implies more bad news is on the way before the investigation is complete.

Given the hasty exit of both the CEO and Chief Financial Officer, I wouldn't look at Monday's stock price decline as a buying opportunity. Shareholders shouldn't panic and sell off their holdings. Investors would be better off waiting to hear the results of Alexion's audit, and seeing which direction the new management team aims to take the company, before doubling down on this beaten-down stock.