Why does pain associated with trochanteric bursitis spread down the lateral leg to the knee?

The IT Band: Because the ilio tibial band runs from the top of the ilium ( the pelvis) to the tibia attaching to a bump called gerdy's tubercle on the outer side of the top of the shin bone. So the trochanter which is the prominence on the top outer part of the femur rubbing on the it band can cause pain radiating the whole distance of the band.

Referral pain. : That is referral pain. However, sometime lower back pain, especially L4 nerve root irritated might induce both lateral upper and lower part of thigh pain.