Why does 4s iphone change to vibrate when I put phone in my pocket? Maybe I'm nuts but it seems my iphone 4s switches from normal (audible) mode to vibrate when I put my phone in my pocket.  Anyone else having this weird issue?
On the left-hand side of your iPhone is this little switch (ring/silent). The plausible answer to your question is, that you turn the switch on/off by accidentally touching it when you put your phone in your pocket. This happens to me all the time as well.

When the switch is turned from on to off your phone will vibrate one time.