The .gov of many states, and thus now apparently a de facto standard, is to test for discharge if the firearm is dropped straight down on the muzzle. So in developing the P320 SIG tested to that "standard" and it passed. Now, field experience (including a SWAT cop in Connecticut who shot himself in the leg by dropping a holstered pistol) has show that if the P320 is dropped at a 30 degree angle, muzzle tilted up and firearm upside down so it lands on the back of the slide, it can (and often does) go off. It's the mass (and attendant inertia) of the trigger mechanism itself that does the dirty deed. There's a striker block, but no trigger block.
The military M17 isn't affected because it uses a different trigger mechanism. It seems this, ah ... shall we say "issue" ... became apparent before or during the military trials so the XM17 used a different trigger. SIG has now issued a "voluntary recall" to repair all of the old design P320s in the wild. The fix involves installing a lighter trigger mechanism, but my understanding is that it also involves some machingin of the slide and received, so there must be more to it than just the weight of the trigger.
Much of the chatter is about whether not SIG knew about the problem and kept quiet, or only found out when they started getting reports from police departments around the country. To me, the fact that the gun they're selling the military uses a different trigger mechanism, coupled with the fact that virtually as soon as the news went viral they had a fix and announced a recall, strongly suggests that they knew and kept quiet.
But that's conjecture on my part.
