I would like to have just a little understanding of why one plate moves north and the other south here.
Does this help, Wordwind:
Three types of movement are recognized at the boundaries between plates: convergent, divergent and transform-fault.

At transform-fault boundaries, plates move horizontally past each other. The San Andreas Fault zone is an example of this type of boundary where the Pacific Plate on which Los Angeles sits is moving slowly northwestward relative to the North American Plate on which San Francisco sits.
Or this from another source:
The San Andreas fault in California is a nearby example of a transform, separating the Pacific from the North American plate. At transforms the plates mostly slide past each other laterally, producing less sinking or lifing of the ground than extensional or compressional environments.