I have a Nexus 5 running Lollipop 5.0.1. I use the built Clock app for an alarm every morning. This morning, the alarm went off, but did not display any UI or turn on the display. The volume buttons, which are set to dismiss the alarm did not work either. I had to unlock the phone, open the Clock app, and dismiss the alarm from there.
What might have caused this behavior and how can I correct it?