Random Indefinite Priority Mode

[Seronac]

I've searched the forums a few times and can find nothing about this. My Nexus 5, with Lollipop, sometimes just randomly and by itself goes into Priority notification mode, set for indefinitely. I've checked the settings and there doesn't seem to be any reason for it to do this, and my phone is in a case that guards the volume controls from unintentional presses. Any ideas what might be going on here, and how I can get it to stop doing this? This whole idea of an indefinite priority mode just annoys me.