Do subsequent pregnancies cause women to carry baby lower? meaning are muscles weaker and therefore don't allow to carry high as normal?

Possibly: There is no doubt that as u age and have more pregnancies it is more and more difficult to maintain muscle definition. This in turn could lead to carrying lower, on a good note labor tends to progress much quicker.

May be: Muscle tone is affected by pregnancies and increasing age. However, there may or may not be a noticeable difference in the height of the uterus between first and subsequent pregnancies.

Yes: The second and further pregnancies are usually carried lower mostly due to stretching of support ligaments rather than weaker muscles strength. This usually is more comfortable as the baby and uterus lie lower and don't press so hard on the lungs to make breathing a problem.