What are some possible reasons that my hips hurt in the back sometimes?

Depends: It may not be your hips. You could have certain arthritic changes or damage to musculature such as your hamstrings that overlay close to your posterior hips. However, more likely, you are having sacroilliac (si) pain that is either painful at that point or pain is being referred there from the si joint. Physical therapy, manipulation, and injections can help get rid of this pain. Best of luck!