Phone turning to vibrate mode automatically?

[drummania]

I am using a Sprint PPC-6800. I have all my class schedules in my calendar. Recently I noticed that sometimes the phone will switch to vibrate mode automatically during some scheduled events, but not on all events. It will not turn back to ring mode after the event.

I wonder what option triggers this switching? I am using the stock WM calendar app and HTC Home 6 tab version.

Thanks in advance.
DM