Combination of several things- number one- he wants it scratched, number two- he trusts you and is " presenting " himself to you.
YOu can do several things- number one- continue- knowing he likes it. Number 2 find another spot ( my guys like their face rubbed and ear scratched etc) number 3- stop petting him when he does it ( but that seems uh a little mean... but he will stop doing it. ) YOu can also have him sit and continue petting him ( he cant present what he is sitting on ya know...)