Situations in which the PCL can tear include -
Both cruciates are necessary for full stability of the knee. Take a look at this illustration. Just look at the brown bone bits and the mauve cruciates in the middle of the illustration. You can see how the ACL and the PCL cross over one another.
If the tibia is pulled forwards the ACL takes the strain.
If the tibia is pulled backwards the PCL takes the strain.