Power seemingly depleted, then restarted...

While capturing video of a multiple alarm fire my iPhone 6s+ Went from 30%+ battery remaining, to showing me a 10% battery remaining box, to shutting off, to rebooting itself. When I travelled away from the scene I notes my key fob battery was also depleted. While driving away from the scene, and after being plugged into my car charger, I saw the red battery icon (full screen) but very shortly thereafter it turned back on and showed 30% battery again. Any idea what is at work here?