First, please forget about blast attack... that's just introducing another moving part to the discussion and is not really relevant anyway... it's been depreciated for years and replaced with an attack flag that's perfectly safe to use. See the manual on attacks.
Now, as to how falling attacks work, and why:
- In order for any of the following to work, there has to be an attack box in the first place. If the falling animation doesn't have an attack, it just can't hit anything. But obviously you don't want an enemy hitting YOU the split second you knock him down with a normal attack right? For this reason when an entity is knocked down, its attack boxes are deactivated by default. That's the way it's been since BOR.
- There is a flag that is activated under certain circumstances. By default, it's whenever the entity is thrown with the native throw system, hit with a "blast" attack, or an attack that has the appropriate flag activated. The latter is how scripted throws do it. When (and only when) this flag is active, the entity's attack boxes are active while it falls.
- Projectilehit determines the type of entities that can be hit while in the above blasted state. If you don't have this set, it defaults to the entity's own type on load for backward compatibility. IOW, throwing enemies into other enemies.
Mixing up any one of these can produce unexpected effects, but they are not bugs.
DC