HotPig....
The P-11's hammer rests on the firing pin. It doesn't weigh very much and it works because it strikes the pin at very high velocity.
If you push on the hammer with the slide down, you will notice that you can push it and the pin forward about 1/16" or so before the hammer contacts the slide rear.
When dropped, the FP moves forward by its' own momentum and is given an additional shove by the hammers'. The slide will also move back a tad which further pushes the FP into the hammer.
Like has been said, 4ft. seems to be the threshold of 'drop danger' with the stock FP spring. If you reduce the hammers' main spring tension a bit, and add the Wolff FP spring, you can move the drop safety range up into the 'shame on you if you make it go off' range.
Slightly modified, you would have been rewarded with a dull 'clunk' when you dropped yours. Glad no one was injured but it WOULD be unsettling, for sure! Also happy that when one of these drop fires does happen, it invariably results in the bullet going straight into the floor. Ricochet injury is still possible so it is not to be taken lightly.