i think it's pretty common for them to crack their backs like that. mine will do it on their own if i a have my hand around their body as they slither through it. you obviously don't want to try and make their vertebrae crack or anything but if it happens during normal handling it's no big deal. that amount of movement and handling will also often stimulate them enough that they deficate, though i've never had any of my snakes pass gas in front of me.