Why are the seas difficult to navigate at the "southern edge" of land?
Fetch.  The longer the wind has to act on the water the larger and more developed the waves become, see this site from the NOAA.  If you look at the southern portion of our planet you will note that there are few land masses to affect the wind or the wave development.  In the Wikipedia entry about prevailing winds you can see that the Southern Westerly winds are only affected by the Cape of Good Hope, Cape Horn, and to a lesser extent Australia.  Contrast this to the Northern hemisphere where North America, Europe and Asia divide the seas into smaller isolated sections.  

There are additional factors involved particularly around the capes.  If you look at the prevailing ocean currents, see global map on this site you will see there are strong currents, driven by the wind, around the southern pole region.  At the capes, these currents become more chaotic and lead to eddies.  Sometimes the eddies make the current flow against the wind.  This results in even larger and less predictable sea states.