Why does my LG G2 pause and display a locked screen with a clock?

[AC Question]

On occasion, my phone will stop after I activate/unlock the screen, or when I am trying to start an app. The screen goes black except for an analog-style clock face in the top third of the screen. The clock is a silvery color and shows the correct time, for what that's worth. There is no ability to interact with the clock, the app, or the phone. It is like a timeout indicator (think hourglass icons on a Windows PC), which is certainly understandable. But why does it occur? Is the OS itself timing out, light garbage collection/memory management? Is it waiting for a signal from a cell tower? Do I have too many apps installed or too much data stored? In other words, do I just have to live with this or can I do something about it?