The last weapon that was inclined to go off if you dropped it was the mass-produced WWII Sten gun, and that was because the bolt operated more or less freely in its cylinder on a spring, due to its design for simplicity and cheapness. So if it fell with the barrel pointing upwards the bolt could move backwards, pick up a round on the way forwards again, and fire it with the fixed firing pin (cheapness again) thus starting the automatic sequence of recoil against spring > forward, collect next round > fire > recoil.....etc.
It is inconceivable that any firearm made since, other than home-made ones, would fire accidentally if dropped, unless it is loaded and cocked with the safety off, and the trigger is somehow operated in the fall, and it is an urban myth that any could.
The weapon in this incident MUST have been loaded with the safety off to have fired. And that says it all about the so-called "training" that these people have evidently received.