Body alignment and movement result from use of muscle pairs. These groups work by opposing each other. Cycling uses hip flexors, for example. Repetitive use from rding makes them stronger. However, if you don't work the opposing muscles, your glutes, imbalance results. The glutes become proportionally weaker. This can lead to lower back pain and sore hamstrings beause the lower back and hamstrings compensate for the weak glutes. Also weak abs can make it worse.
Core work, and overall cross training, keeps things in check.