Why do women get back pain while pregnant?

Many reasons. Strain, arthritis, herniated disk, sciatica , urinary tract infection or obstruction, kidney stone, baby position among a few others.
Subluxation. The most common complaint in pregnancy during the 3rd trimester is low back pain. As the uterus expands and the abdomen protrude, your center of gravity moves backward. The common "duck waddle" walk of a pregnant woman is to compensate that center of gravity. The lumbar spine can subluxate (move) which is exactly what chirpractors treat. A good chiro in pregnancy is worth their weight in gold.