Will a dislocated knee also cause a PCL tear most of the time?

Yes. Pcl tears typically result from a dislocated knee. In knee dislocations, multiple ligaments are torn including the acl, pcl, and one of the collateral ligaments. Isolated PCL injuries are somewhat rare and usually occur from a direct blow to the top of knee such as a dashboard impact during a car accident or a fall directly onto a bent knee.
Yes. The PCL is typically torn whether there is an anterior or posterior knee dislocation. There are a few rare reports of the PCL being intact following a knee dislocation, but they have been reported because they are so rare.