What could cause the knee joint to painfully pop after having your leg straight? why would someone walk straight legged?

If you are moving from a bent knee to straight, the pop could be any number of things including a tendon or ligature. If it continues, it should be evaluated, especially since it is causing pain. If it hurts to bend the knee, then a person would try not to do this and walk with a straight leg-this too should be evaluated so that a diagnosis can be made and treatment given. May need to see ortho.