Let us assume that the battery has enough power. Here is my assumptions.
1) The battery locking machanism fails causing the battery to eject from its position. Cause: Owner fails to make sure thai the battery is securely locked or the locking machanism damaged.
2) The power distribution circuit or firmware fails tempolarily or permanently.
3) Spark flight controller firmware fails under a certain condition and causing the system to reboot or stop mid flight. And that cause the motors to stop spinning.
4) Spark receices motor shut off command from mobile/rc mid-flight by pilot mistake or mobile device program error especially running Dji Go 4 using device not in compatible list.
5) ESC/motor/propeller failure.

1. I'm not sure on that, I always preflight check my drone. 34 flights (20,000ft) no issues, and on my 35th, 2 minutes into the flight, it fell.
2. possible.
3,4,5 doubtful. its not the motors that stopped on mine. it shut down. the rc shows no connection, no camera image sent to phone, and the video that was recording stopped. it lost power, not just motor shut off. if you sent a csc, you'd be able to restart the props and possibly save it. mine blacked out, and dropped. not just the motors, all power and all signal.  using an iphone 6s, latest version of dji go, latest firmware on battery, rc and drone.

there is something more going on here. I was well within operating temperatures, and never had an issue with it. on a normal flight I've done at least 20+ times.  it's like it loses all power to everything.