The pilot reported that, during preflight inspection, after a passenger had entered the airplane, he ensured that the right-rear cargo portion of the right cabin door was secure. The right side cabin door was a two-piece forward passenger door and rearward cargo door combination, and was adjacent to the second row of passenger seats. He did not close the right-front portion of the door because the passenger had secured that door multiple times before. After completing the preflight inspection, he entered the airplane and observed the passenger close and latch the forward portion of the right door. He then saw the passenger, who was seated in the right rear passenger seat struggling with the seatbelt which passed between the seat and the cargo door on the right. The pilot suggested the passenger switch seats, and the passenger moved to the left rear passenger seat and fastened his seat belt. The pilot reported that, during the flight, while maneuvering and descending to 1,600 ft mean sea level, he banked to the left and heard a "brief whistle", followed by a "loud boom sound." He looked behind him and saw both portions of the right cabin door open. He returned to the departure airport without further incident. The airplane sustained substantial damage to the fuselage. Postaccident examination revealed red paint transfer on the forward portion of the door from the rear portion door latch and that both door sections could be opened if the latch for the rear door was open. The pilot reported that he believed that the right rear passenger seat belt "must have pulled on the [rear] door latch enough to dislodge it." He added that the red paint transfer was not there before the flight. The pilot provided a postaccident examination video, which showed that he was able to use the larger airbag section of the seatbelt to wedge open the rear cargo door latch. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. The "Rear Cargo Door Emergency Exit" placard in the airplane stated: 1. Open front cargo door as far as it will go. 2. Push rear door handle forward and force door open.