For Vikram Pandit, a trip to Tokyo for the International Monetary Fund and World Bank conference last month seemed routine. But Michael Corbat, the longtime Citigroup executive who joined Pandit there, knew better.

Unbeknown to Pandit, Citigroup chairman Mike O'Neill had told Corbat that the board could seek Pandit's resignation as chief executive and hand the job to Corbat, according to people familiar with the situation. A day after Pandit and Corbat returned to New York, that is exactly what happened.