Some guns won't fire if they're dropped. Glocks won't.
However...all guns contain a cartridge, which has the bullet, gunpowder and primer in a little brass container; a firing pin; a trigger; and some sort of spring loaded mechanism. When you pull the trigger the spring loaded mechanism hits the firing pin, which ignites the primer, which ignites the powder, which makes the bullet fly out the end of the barrel.
A gun that will fire when it's dropped is able to exert enough force on the primer without the spring-loaded mechanism hitting the firing pin to set it off.
Virtually all modern handguns will not fire when dropped. If they can, there is a design flaw, and they are recalled so that this can be corrected. This is what happened with the SIG P320 three years after its introduction in 2014. Note that it took three years for the problem to manifest itself; the pistol had to be struck from the rear at a precise angle to make it fire.
The mechanics of drop-safety (or the lack thereof) are somewhat complex, given the variety of firing mechanisms found on firearms in the past 100 years or so, and the correspondingly wide variety of safety mechanisms. The bottom line in all these tragedies is that something has caused the firing pin to hit the primer without the trigger being pulled.
The worst culprits for this were the old cowboy single-action revolvers (and some early replicas thereof), which could easily fire without the trigger if something struck the large, prominent external hammer hard enough. For this reason, it was (and still is) recommended to carry these with only five cartridges loaded (in your "six-shooter) so that the hammer rests on an empty chamber. All modern replicas have since added safety devices to these old designs to make this unnecessary, but earlier models are still out there. This could have been one of those.
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