Generally this happens if the external power source is interrupted for any reason, even very briefly like you have a loose connection and then you hit a bump in the road. Then the Garmin tells you it's shutting down (even if it's own battery has plenty of charge) and gives you 15 secs to cancel this. If you don't see this notification in time, it switches itself off. When the external power comes back on, the Garmin switches itself back on.
This, to me, is Garmin's (all Garmins) most annoying feature. It cannot be switched off. A mobile phone (and maybe other brands of GPS*) cam cope with with a an on/off external power supply without complaint.
* If you know of one, I'd love to know.