One claim is that the gun can fire when dropped at a certain angle from a certain height. The voluntary "recall" lets you send it back to Sig and they replace some parts. I think the cause was because the trigger itself was bulky enough for a drop to give it enough inertia to fire, but I'm not 100% sure on that.
The other claim is that the P320 can fire without being dropped, and while holstered, seemingly on it's own. That's all I really know about it.
I own a P320, and I consider it an unsafe weapon at this point. I have not had the self-recall fix done and I'll never chamber a round in it again, so I guess it's a paperweight now.