yeah, it could be different for you ... but somehow it kept happening on mine. Because i was on a call for few times and after the call I check my new msg and It locked me in portrait mode.

Edit: i just tried it again ... and it's not working for me either. So i guess it must be something else. But for the few times that it happened , it was exactly how i posted. Unless there's something I must have done that I don't remember.