I think one of the things that other posters are alluding to is that the phone can experience very significant forces if you are dancing, especially if it is being held in something like a purse or other bag that's flying around wildly as you dance (or if the phone is dropped). I'm pretty sure that if I threw my phone in a bag and caught it by the strap, it would experience far more G force than in any ordinary car crash, and if it is surrounded by screaming and general chaos, I think it's making a fairly understandable decision.

The sensor fusion algorithms get confused by this, and the phone decides to call 911. It displays a notification for 10 seconds, then it starts a further 10 second countdown where it buzzes at maximum power and plays a siren noise at maximum volume. Under normal circumstances a reasonable person would notice these very loud alerts, and cancel the call. In a festival scenario where the users are deafened by other noises and possibly drunk/high, you might not notice. The phone then makes a call to 911.

How would you alternatively design a system that calls emergency services in a crash? Where would you personally choose to calibrate the false positive/false negative rate? These are hard design choices, and there will always be incorrect detections with any automated system like this.