Krispy Kreme, Opendoor, Rocket and Kohl’s are merely the public face of a summer boom in bets by private traders.

Bursts of euphoric trading are often bullish for near-term returns, but rapid surges in speculation also raise the risk of a pullback, Goldman said.

Jackson first grabbed Wall Street’s attention 10 years ago, with a 99-page presentation to Yahoo Inc.’s board on why the tech media company should oust its former head Marissa Mayer due to ...