Groupthink needs no conspiracy. In a trading room where everyone is long, the person who sees a problem has to weigh being right against being the one who ruins the mood. Most people stay quiet, and silence reads as agreement.
The result is a room that sounds like confirmation but contains one idea held by many people. Positions correlate, risk concentrates, and the unwind hits everyone in the group at the same moment.
Structure beats willpower here. Assign someone to argue the other side, collect views in writing before discussion, and treat unanimous agreement as a warning rather than a green light.
Related: herding, social-proof, echo-chamber, signal-group