from Wikepedia:
Denver apparently lost control of the aircraft while attempting to manipulate the fuel selector handle, which is located in the rear cockpit, after running out of fuel in one tank. Witnesses stated that the plane made a sudden pitch-down plunge into the water, leading to speculation that, in reaching around to the rear, Denver bumped or kicked the side-stick control.
Denver had recently purchased the aircraft and had only had about a half-hour orientation flight the day before the accident. The NTSB cited Denver's unfamiliarity with the aircraft and his failure to have the aircraft refueled as causal factors in the accident. Denver was the sole occupant of the aircraft. Prior to the accident, the FAA found that he did not meet the medical standards prescribed in Part 67 of the Federal Aviation Regulations due to his ongoing drinking problem, and a determination was made that he was not qualified for any class of medical at the time. At least a third-class medical was required to exercise the privileges of his pilot certificate