Monty (my BP) did that once he had some stuck shed and i had him wrigling thru a wet towel in my hands and ON HIS OWN i felt more that hear his back kinda go pop pop pop and i was really scared but then he strangled a mouse 15 minutes later and i figured if he could still do that hes fine
im sure its just like cracking ur knuckles or ur neck which i do all the time