Why does Windows Mobile changes to vibrate automatically?

[tahlsr]

I remember someone once saying that maybe this had to do with appoinments you had in your calendar. i.e. the phone is being slick by going to vibrate mode when you are in an appointment. See if your appointments correspond to when it goes into vibrate mode. Another question (cause I've had this issue too) is does this only happen to people with the HTC Home plugin?