During the trial of United States v. Nixon, Nixon had committed the act of obstruction of Justice and abuse of power, and contempt of congress. Nixon had stolen tapes for campaign purposes. This is known as the watergate scandal.
The judicial branch knew that this was against his power as president and would take action against him. So the Judicial branch would take three articles of impeachment against Nixon. Like stated above these were Obstruction of Justice, abuse of power, and contempt of congress. This is what lead to him stepping down from office so that he wouldn’t be impeached.