Monday was a good day on Wall Street, as major benchmarks largely added to their big gains from last week on continued optimism among investors. With the U.S. having chosen not to go forward as planned with new tariffs on Mexican goods, market participants seemed to be more comfortable with the idea that economics and geopolitics might not spell inevitable trouble for stocks globally. However, some companies had bad news that sent their share prices lower. Coeur Mining (CDE -1.53%), Spark Therapeutics (ONCE), and Tandem Diabetes Care (TNDM -4.29%) were among the worst performers. Here's why they did so poorly.

Coeur gets tarnished

Shares of Coeur Mining fell 5% on a bad day for the precious metals market. Silver prices were down more than 2%, in part because investors have been using gold and silver investments as a hedge against geopolitical risk. With the trade situation looking a bit less confrontational, silver prices gave up some of their gains, falling about $0.35 per ounce to $14.60. Coeur itself has dealt with some production problems that have held it back even relative to some of its silver-mining peers, as challenges at its Palmarejo mine in Mexico had a substantial impact on its overall numbers. Going forward, Coeur will need both to execute better and to see higher silver prices if it wants its stock price to get back to levels it saw a few years ago.

Silver coins and bars piled haphazardly.

Image source: Getty Images.

Spark's deal gets delayed

Spark Therapeutics' stock suffered an 8% drop following an announced delay in a key strategic transaction. Roche proposed a $4.8 billion buyout of the biotech company back in February, and investors have been optimistic that the deal will go through as planned. Yet regulatory overseers at the U.S. Federal Trade Commission said that they'll need more time to examine the agreement, and the Competition and Markets Authority in the U.K. followed suit with its own look at the deal. Roche is still optimistic that the deal will get done, but investors worry that if it can't get approval within the next couple of months, it could lead Roche to reconsider the acquisition and result in Spark losing all the gains it saw following the initial offer.

Tandem sees shares fall

Finally, shares of Tandem Diabetes Care lost nearly 7%. The move came despite the fact that the diabetes treatment specialist announced positive results from two studies of its t:slim X2 insulin pump, which Tandem sees as "a major breakthrough by the company in the field of diabetes management." Users of the pump saw significant improvements in hemoglobin A1c levels and reductions in mean glucose levels, prompting the conclusion that the pump improves glycemic control and is convenient for patients. Yet it's possible that investors responded negatively to a case of diabetic ketoacidosis during the study due to an infusion set failure. If further studies confirm positive results without the same concern, then Tandem could easily regain its lost ground.