If you go past 18-20 volume units when the engine is off, the NAV will restart because it's not receiving enough power. I've experienced the same issue with my 2013 Civic navigation unit. If I go past 20 units when the engine is on, it does not turn off. I took my car to Honda Center Peshawar Road and they told me all this.
I don't know if this is relevant, but I've also noticed that the navigation unit turns off sometimes when there is a sudden, deep bass note.