Nice looking snake you have there.

I would gather that behavior may have something to do with heat and O2 exchange. Though boas (BC's) don't have visible heat pits, they do have heat sensing receptors under their scales. There are various studies on that subject. Obviously our head area gives off heat and oxygen which would give the snake some type of cue to check things out if they were curious.

I see that behavior from my BC and my coastal carpet and to a lesser extent in the royal.

I think maybe the semi arboreal snakes like going up, and that's usually where our faces are in relation to them.

That said, it could just be an individual behavior that varies from snake to snake.