Why does my phone vibrate for alarm?

[iMore Question]

My alarms will randomly vibrate instead of make an audible sound when the alarm goes off. The mute switch is not in the off position and the ringer is also not low or on mute. This does not happen all the time. One day it works fine and the next it doesn't.

 

[mtmasean]

Additional info: this is an I-phone 6 Plus. The alarm and my ringer is set to ring and vibrate at the same time. I also have the flash set to go off when the phone rings.