The stock market kept performing well on Monday, and the Dow Jones Industrials posted its 12th straight all-time record high. Market sentiment was sunny following the release of Warren Buffett's annual shareholder letter over the weekend, and the Oracle of Omaha made positive comments about the sustainability of the current bull market, which was able to overcome early declines in the major market benchmarks. However, some stocks didn't fare as well as the overall market, and Tesla (NASDAQ:TSLA), AmTrust Financial Services (NASDAQ:AFSI), and Horizon Pharma (NASDAQ:HZNP) 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.  

Tesla gets a poor review

Shares of Tesla fell 4% after the electric car manufacturer received a downgrade from analysts at Goldman Sachs. The Wall Street giant cut its rating on Tesla from neutral to sell, citing concerns about the company's ability to launch its Model 3 mass-market sedan, execute on the strategy behind its acquisition of solar specialist SolarCity, and deal with the need to tap the capital markets to raise cash. Yet even the bearish analysts admit that Tesla has a competitive advantage over rival automakers with respect to the technology and architecture of its electric vehicles, and Tesla's efforts to boost battery production appear to be working. Nevertheless, the question facing investors is whether Tesla can execute well on the huge opportunity it has in front of it, and at least in Goldman's eyes, that could prove more difficult than initially thought.

Tesla electric vehicle.

Image source: Tesla.

AmTrust deals with a potential problem

AmTrust Financial Services' stock plunged 19% in the wake of the company's fourth-quarter financial report. The insurance provider said that results for the quarter looked solid, with substantial rises in premium volume and service and fee income, as well as a pickup in net income of nearly two-thirds compared to the year-ago quarter. However, investors appeared nervous about AmTrust's comments about the timing of its annual report filing, which included a statement that the company expects to disclose "material weaknesses in in its internal control over financial reporting" that existed as of the end of 2016. Until AmTrust reveals exactly how material the impacts might be and how it expects to fix them going forward, shareholders are likely to remain cautious.

Horizon disappoints investors

Finally, shares of Horizon Pharma fell 6%. The biopharmaceutical company said that sales were up 27% from year-ago levels, but it posted a net loss of $130.5 million. Even after adjusting for various extraordinary items, Horizon's bottom-line growth was limited to less than 1%, resulting in flat adjusted earnings per share from the year-earlier period. Yet even expectations of 19% to 23% top-line growth in 2017 with 12% to 22% increases in pre-tax operating profit weren't enough to satisfy shareholders. Nevertheless, Horizon CEO Tim Walbert remained optimistic, noting that "continued commercial execution and the completion of two transformative acquisitions [will] bolster our rapidly expanding rare disease business."