A few points to consider:
During a jump-run the 'jump-master' (skydiver in charge of the lift) will open the AC door 1-2 minutes before the expected exit point (according to the GPS).
On a L410 the (fairly heavy) rear door swings inward and slides back into the tail section. There are not usually any kind of shock absorbers at the end of the door tracks so care must be taken.
Of the few drop zones still using this AC most consider it to be 'full' with one pilot and 16-19 jumpers (including parachute equipment, each parachute weighing 20-30 lbs).
I don't wish to speculate about exact cause of the incident but these factors may well have contributed to it.