Why does my Android 5.1 phone launch in vibrate mode?

[AC Question]

Recently, my Android 5.1 phone began starting up in vibrate mode, rather than the normal sound mode. Is there some setting I can change? In the 1.5 years I've had the phone I've rarely put it in vibrate mode and I expect that if it's in sound mode when I turn it off, it will be in that same mode when I turn it back on.