LG G2 sound randomly turns on from vibrate mode

I have a Verizon LG G2 (VS980) updated to Android 4.4.2, which I always keep on vibrate mode.

However, every day, at exactly 3 PM, without fail, it turns itself from vibrate back to sound. It doesn't matter whether or not it's charging, if it's on wi-fi or data, or anything else.

Any idea what may be causing this, or how I could fix it?