The stock market had a solid performance on Monday, with the Dow Jones Industrials and other major benchmarks climbing to record highs. Progress on the House tax bill, encouraging signs from President Trump's Asia trip, and a lack of negative economic news contributed to general enthusiasm from market participants. Yet even with a positive mood on Wall Street, some companies weren't able to share in the good news, and Tivity Health (NASDAQ:TVTY), Pandora Media (NYSE:P), and Weis Markets (NYSE:WMK) were among the worst performers on the day. Below, we'll look more closely at these stocks to tell you why they did so poorly.

Tivity looks less healthy

Shares of Tivity Health plunged more than 30% in the wake of a competitor's move to threaten one of its key programs. UnitedHealth Group (NYSE:UNH) has expanded its own fitness benefit offerings for seniors under its Optum unit's Medicare Advantage plans, which investors saw as a direct challenge to Tivity's SilverSneakers program. Some saw the sell-off as unwarranted, with analysts noting that SilverSneakers will retain its exclusivity in key markets as well as Tivity's existing relationship with UnitedHealth in offering fitness facility access under its Prime Fitness network. Given Tivity's success over the past year, it's also possible that shareholders are simply taking profits after a long, lucrative run for the senior health specialist.

Tivity Health logo.

Image source: Tivity Health.

Pandora keeps falling

Pandora Media stock dropped over 5.5%, adding to its losses from late last week following the release of the streaming music company's third-quarter financial report on Thursday evening. Poor results and downbeat guidance sent investors running from the stock on Friday, with the company saying that difficult conditions are likely to continue in the advertising business in the immediate future. New CEO Roger Lynch remains optimistic about Pandora's long-term prospects, but a full turnaround could require costly investment that will dramatically increase the amount of risk involved for investors. With Pandora focusing its efforts on its core market after having pulled back from expansion efforts abroad, now is the time for the company to prove it can remain viable going forward.

Weis Markets sinks

Finally, shares of Weis Markets declined 16%. The supermarket chain reported its third-quarter results after the market closed on Friday, which included a 15% rise in revenue on a 1.5% increase in comparable-store sales. However, net income dropped by more than half from year-earlier figures, which Weis blamed on a tough promotional and pricing environment, price deflation in key value-added areas, and difficulty with managing inventory. Investors are nervous about the rising levels of competition in the grocery industry, and although Weis is small enough to be a potential acquisition target from a larger rival, shareholders still have to be careful that the company maintains healthy enough fundamental business performance to attract attention from would-be buyers.