You can Drop Prone as a Free Action. This happens on your turn.
You also drop prone if suppressed and can roll Mobility to drop if a grenade lands in your hex. These happen outside your turn and are not reactions or actions
I believe you also drop prone if an explosion goes off in your hex, or if you take a Critical Hit to the Legs. These happen automatically.
I think the problem is that there are so many situational exceptions to "Drop Prone is a Free Action, but only on your turn, except when suppressed, or explosion, or...etc" Maybe just make Drop Prone a Free Action you can do at any time as a reaction, much like you can Block at any time as a Reaction. Having a rule with so many exceptions doesn't really help the game and slows down play
I guess the question is why is Drop Prone only something you can do on your turn? What does it benefit the game to be that way? The only thing I can see is that it becomes a defacto "dodge" attempt... but it still robs you of a Fast Action on your next turn to negate (i.e. stand up), so I still don't see the problem.