The whole thing does not make sense. In a 109, you cant simply climb from the cockpit into the cabin, so the copilot must have been at the controls all the time. He was maybe just looking into the cabin to monitor the jumpers.
The report is innaccurate - the sliding door is for the cabin, the pilots door is hinged. There is simply no reason why the pilot would be unstrapped or deliberately have his door open. I cant think of any way this could not be a deliberate act.
Unless - what actually happened maybe is that the report is completely misleading, and one of the pilots was in the back acting as the jumpmaster throughout, the other was at the controls throughout, and it is the one in the back who fell.