Gee, I don't know -- maybe because an 8 pound bowling ball is sitting on your spine! That does tend to make things uncomfortable. While the seat cushions do help, nothing works like getting the weight off your spine, like being submerged in a pool -- heaven. Anything that changes the baby's position and gets his big butt off your spine makes you feel better! Sometimes, lying on your side helps, or getting on all fours will also take the pressure off.
For some women, the only thing that helps is giving birth and getting the little rascal out of there.