Why would having my sacroiliac joint adjusted make my knee feel better? My orthopedic doctor sent me to a physical therapist who manipulated my hip and now my knee pain is much better. I don't get it.

Researchers who study the question you asked don't have an exact answer yet. A recent finding among several studies confirmed a definite relationship between patellofemoral pain syndrome (PFPS) of the knee and hip motion and strength. They found that a difference in hip internal rotation from one side to the other seems to contribute to knee pain. That difference was also accompanied by decreased hip muscle weakness.

Moving the sacroiliac joint (SIJ) through an adjustment or manipulation may have several beneficial effects. There may be a reflexive change in muscle activity at the knee joint. Electrical activity in the muscles may decrease leading to pain relief from a muscular source. It's possible that a stimulus of any kind makes a difference.

There's some evidence that manipulation has a direct effect on the central nervous system to reduce pain. More than likely there are many factors that combined together influence the SIJ-knee dynamics. More Further studies are needed to sort this all out.