So:
1. The gun was loaded.
2. The round was chambered.
3. The safety was off.
4. The impact of the backpack hitting the floor was enough to jar the gun into firing all by itself (????)
or
4. The dropping of the bag caused ‘something’ in the bag to get inside of the trigger guard and pull the trigger with enough tension to make the round fire.
All of this and it wounded not one, but two people.
I totally see how this could have happened.
Firing pin safety
And of course the trigger safety. Glock's `Safe-action' three safeties are virtually idiot-proof from half-wits, but there are a lot of complete idiots in this rich tapestry we call life.