Living in South Africa I see these systems quite often. I haven't had the chance to look at the history of the system but I'm assuming it's moving from west to east, given the season.
Remember it's the dead of winter down here right now, and while that area is tropical, water temperatures are no doubt way below the summer temps, probably around 20-24'C.
In winter we get 'cut-off lows' which move from west to east, often developing off the east coast of South Africa, these are typically cold core systems, though occasionally they have an impressive circulation with associated convection. Though these west to east moving systems will never have a symmetrical shape. These storms can often occur around Cape Town as well, where I am located, though as they are brought NE for a while (before typically moving SE with the jet stream) they may develop some more convection over the center as they enter slightly warmer waters (waters around Cape Town are about 12-15'C in winter)...
Here is an image of a cut-off low situated off South Africa in 2001 -

These systems differ from the tropical activity that we get in the months of February through May in that those systems tend to develope east of Madagascar and then move west or south. These winter cut-off lows are often associated with cold fronts and thus come from the opposite direction and move from west to east.
To answer the question about how common they are, there are several each year. Some look more tropical than others.