I think most people do not take the "Forward Collision Warning" system into account. I think that's the main reason of PB.
I have noticed on day one "Forward Collision Warning" will come on if you follow another car and that car's brake light comes on and if you don't release you acceleration paddle immediately. This often happens at intersections when the car in front slows down to take a turn. This dinged my safe score when the car in front slows down to turn into a shopping center. From my experience my speed should be OK to miss him when my car arrive at the turn in. But not the Tesla, it sense my foot is still on the acceleration paddle!!
Also, the faster you travel, the easier is for the system to detect false distances. So I try to slow down on 2 lane highways and not use cruise control as the system don't slow down to take curves.
Another thing about overtaking on cruise control. If you are traveling at speed limit and overtake by accelerating over cruise limit (very easy as Tesla is fast!!), The system will warn that it will not brake and FCW may come on. I have that happened to me on I280 near San Francisco. Scared the hell out of me! So now I disengage AP when overtaking at speed.