Seems that we'll likely never know what really happened....regardlesss......it also seems like a good time to review safety bits for those P99 and PPQers out there (myself included). We should make this into a learning opportunity for those new to the P99/PPQ.
GENERAL - One should periodically check the integrity and function of the safety mechanisms of your pistol(s).
With the slide off, make sure the striker safety plunger moves freely in its channel and is not excessively lubricated. The striker safety plunger should depress downward and quickly return to rest without excessive force from a finger or probe.
With slide off, make sure the striker safety plunger effectively blocks forward movement of the striker by gently pressing forward on the striker's lug until you feel the striker hit the body of the striker safety plunger.
With the slide off a P99, make sure the trigger bar is immobilized by the pivoting trigger by gently tugging rearward on the trigger bar with the slide off.
With the slide off on a PPQ, make sure the trigger dingus effectively immobilizes the trigger and trigger bar through a similar method to above.
To be especially thorough, you can even reassemble and check an EMPTY gun by shining a flashlight down the magwell to watch the trigger bar's interaction with the striker safety plunger upon actuation. The striker safety plunger should not be engaged until the trigger pull has begun.
Failure of any of these tests should see the pistol removed immediately from service and the manufacturer contacted.
Lastly, it should be noted that these safety systems are similar on all modern striker fired guns, and the above checks (modified accordingly) should be done periodically regardless of brand/type/make/etc.