First Random Turn off

[JC4131a]

So I was turning on my mobile hotspot for the first time and I connected to it with my laptop and then I walked away, came back, and there was no internet. I checked the phone and it had turned itself off despite having tons of battery left. It wasn't plugged in when this happened and I now have it plugged in and working fine. Is this a known bug or just something random that happened to me?