As you lose control and range of motion in segments of the spine (especially turning the ribcage), the rest of the spine starts to lock up as well, then you get the majority of your motion from the hypermobile segments. When people come to class to stretch the back, they may not be aware that they keep overstretching one particular area making it more unstable, while tightening other areas of the spine that are supposed to move. 

In ALL classes, we cover a myriad of methods to sense your areas that may be overused or under-protected.