Why women with naturally big boobs ends up having surgery for reduction?

Depends on how bad their backpain is. Consider that sometimes there are other underlying conditions that the extra breat weight just makes worse, such as scoliosis and lumbar herniations. In these cases it's either get the breasts reduced, or walk around all day with an anatomical vest.

Either way, having breasts that are too large are a pain to have when you work by moving around or you have to care about your children. You can't even sleep on your back or on your tummy, because you'd either crush them under your weight or they'd fall to the sides.