At 0745 Hawaiian time, the airplane sustained substantial damage when its tail impacted the ground after a ramp agent opened the passenger door and boarded the airplane. The captain, first officer and eight passengers aboard the Part 135 passenger flight were not injured. Visual conditions prevailed. After passengers were boarded, the second-in-command (SIC) noted an error in the load manifest and communicated it to the ramp agent. As the ramp agent opened the aft passenger door and boarded the airplane, the pilot-in-command (PIC) exited the airplane and, according to the operator, the airplane “tilted downward” and the tail tie-down assembly struck the tarmac, sustaining substantial damage to the aft bulkhead (FS 427.88) and tiedown assembly.