Probably because the back is fleece and the tugging of the birthing caused stretch.......how much of a bubble is there.....maybe if you do the quilting with the backside " up" you will be able to ease in the fullness....move slow but fast stitching will suck in some of that fullness. If I'm going to birth something, big or small, I make the back just a smidge shorter all the round, then when turned right side out it sort of curls the edges to the back and no peekaboos
Just my way...