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Well it was commodities, not stocks, orange juice specifically. Here is the explanation from Wikipedia.

…Watching a television news broadcast, Winthorp and Valentine learn that Beeks is transporting a secret United States Department of Agriculture (USDA) report on orange crop forecasts. Winthorpe and Valentine recall large payments made to Beeks by the Dukes. They realize the Dukes will obtain the report early to corner the market on frozen concentrated orange juice.

On New Year’s Eve, the four board Beeks’ train, intending to switch the original report with a forgery that predicts low orange crop yields. Beeks uncovers their scheme, and attempts to kill them, but is knocked unconscious by a gorilla being transported on the train. The four disguise Beeks with a gorilla costume and cage him with the real gorilla. The group deliver the forged report to the Dukes in Beeks’ place. After sharing a kiss with Ophelia, Winthorpe travels to New York City with Valentine, carrying with them Coleman’s and Ophelia’s life savings to carry out their plan.

On the commodities trading floor, the Dukes commit their holdings to buying frozen concentrated orange juice futures contracts, legally committing themselves to buying the commodity at a later date. Other traders follow their lead, driving the price up; Valentine and Winthorpe short-sell juice futures contracts at the inflated price. Following the broadcast of the actual crop report and its prediction of a normal harvest, the price of juice futures plummets. Valentine and Winthorpe buy at the lower price from everyone except the Dukes, fulfilling the contracts they had short-sold earlier and turning an immense profit. … When the Dukes prove unable to supply the $394 million (equivalent to $1.01 billion in 2019) required to satisfy their margin call, the exchange manager orders their seats sold and their corporate and personal assets confiscated, effectively bankrupting them. “Turn the machine back on!”