Vibrate changes to Silent

[showtime711]

When I set my phone to vibrate, inevitably at some point I'll find out I'm not getting my calls and when I turn on my TP I see that the ringer and system is set to silent. Am I accidentally hitting one of the buttons or a button combination that's a quick key to turn the phone to silent or something? If the phone is in sleep mode, pressing any button will not do anything, only the power button, correct? I frequently take the phone out of my side holster during the day and maybe I'm accidentally hitting button? Is this happening to anyone else?