My phone randomly deletes all music.

I have a 128Gb iPhone 6S. IOS 9.3.1. I do not buy music from iTunes. I have all my music on a hard drive, and iTunes is set not to sync. Ever. I do not use Apple Music, did not sign up and only set the phone to show music on the phone.

Yet twice now my phone has randomly deleted the whole of the music library on the phone (almost 10,000 songs) whilst I was not connected to iTunes.

Does anyone know why my phone would do this, and how to stop it happening again?