One of mine actually does yawn fairly often even when it hasn't eaten in a while. No idea why, but its pretty normal I guess.
I've read somewhere most snakes would realign their jaws in that way but some species including BP's yawn at other times too, but no reason why.
and I like it that my BP does it since it allows me to check for mucus or any other signs of RI when it does. I feel you have nothing to worry about, but then again I'm probably one of the least experienced around here