Monday was one of the strongest days in stock market history as investors responded positively to the idea that the U.S. would be willing to talk to China about their trade differences. The Dow was up more than 600 points, and most major benchmarks finished up between 2% and 3% on the day. In general, gains were broad-based throughout the market. Yet a few stocks still lost ground. KB Home (NYSE:KBH), Longfin (NASDAQOTH:LFIN), and Protagonist Therapeutics (NASDAQ:PTGX) were among the worst performers on the day. Here's why they did so poorly.

KB Home gives back some gains

Shares of KB Home dropped 4% as the homebuilder reversed course from gains on Friday. The company's fiscal first-quarter financial report came out late Thursday, and it included a revenue increase of 6% on a 7% jump in average selling prices to $389,000 per home. After adjusting for tax reform and other one-time items, adjusted earnings almost tripled from year-earlier levels, helping to send the stock higher by more than 2% on Friday. Yet today's pullback reflects slightly less certainty about the immediate future for homebuilding, with factors like rising rates potentially playing a role in holding back KB Home's ability to keep taking maximum advantage of opportunities to serve would-be homebuyers.

Two-story house with wood-beam highlights and a green L-shaped lawn.

Image source: Getty Images.

Longfin gets called a lemon

Longfin stock plunged nearly 17% after the blockchain and cryptocurrency specialist became the latest target of short-selling specialist Andrew Left at Citron Research. Citron tweeted Monday morning that Longfin "is a pure stock scheme," adding that the "[SEC's enforcement division] should not be far behind" after alleging inaccuracies and fraud in Longfin's filings and press releases. Longfin has had a wild ride in the roughly three months in which its shares have traded, and even after today's drop, the stock is still up more than 1,000% from its $5 IPO price in December. Yet with bans on cryptocurrency advertising getting implemented, some see Longfin's future as a lot less certain now.

Shareholders get antagonistic about Protagonist

Finally, shares of Protagonist Therapeutics lost more than half their value. The clinical-stage biotech announced that it would discontinue a phase 2b study of its PTG-100 treatment for ulcerative colitis, because the early analysis led the independent data monitoring committee to determine that it would be futile for the study to continue. Protagonist also said it would postpone decisions about whether to start a phase 2/3 trial of the treatment in a population of patients suffering from chronic pouchitis until it has been able to review the study data fully. With PTG-100 on hold, investors are worried that they'll have to wait a long time before the rest of Protagonist's pipeline can get closer to a potential approval.

Dan Caplinger has no position in any of the stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.