STOCKS FALL: Here's what you need to know

Stocks had the biggest drop in a month on Monday, as the Dow sank back into negative year-to-date territory, and treasury yields jumped towards the highs of 2015.

First, the scoreboard:

• Dow: 17,703.62, -206.71, (-1.15%)
• S&P 500: 2,074.71, -24.49, (-1.17%)
• Nasdaq: 5,085.63, -61.49, (-1.19%)

And now, the top stories:

1. One tweet sent a stock down 25%. Shares of Dublin-based specialty pharma Mallinckrodt nosedived after the firm that exposed allegations about Valeant Pharma tweeted that they could have even more downside. Andrew Left of Citron Research, which sent the tweet, told Business Insider, "The market has been so focused on Valeant that they forgot about other platform companies who are levered and face the same headwinds in reimbursement." Valeant shares have dropped 45% since Citron released its report on Valeant's relationship with Philidor on October 21. Valeant is holding another conference call at 8 a.m. ET Tuesday, after saying the call by hedge fund manager Bill Ackman was too "scripted".
2. Canadian Pacific Railway may acquire US counterpart Norfolk Southern, Bloomberg reported, citing people familiar with the early-stage talks. Canada Pacific is the country's second-largest railroad. Ackman acquired about 14% of its stock and removed its board a few years ago. It is reportedly raising financing. Canadian Pacific shares rose by as much as 5%, Norfolk Southern jumped 12% after being halted.
3. SeaWorld will reportedly phase out killer-whale shows in San Diego. The San Diego Union-Tribune reported that the marine-mammal park would replace the shows in 2017 with a new orca experience that's "'informative' and designed to take place in a more natural setting that would carry a 'conservation message inspiring people to act.'" The company is still cleaning up its image following the 2013 "Blackfish" documentary that detailed the psychological travails of the amusement park's animals.
4. The Fed's Labor Market Conditions Index was 1.6 for October, up from 1.3 prior, and beating the forecast for 0.9. The index reflects 19 labor-market indicators. It was released as usual on the Monday after the jobs report; Friday's release showed the US economy added 271,000 jobs last month, the biggest gain this year, while the unemployment rate fell to 5%, the lowest since April 2008.
5. The New York Fed's monthly Survey of Consumer Expectations showed that consumers anticipate spending more. The median household expected spending to increase by 3.47%, up from 3.18% prior. And, the median consumer's expectation for inflation one year ahead was 2.82%, above the prior reading of 2.73%.

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