I am sure other Forum posters may have a better reply for you than I have, but for starters, one might ask why SAA’s aircraft spend all day on the tarmac after they arrive in Europe ?
Also it’s the same scenario in Sydney where both BA and VS park their planes on the tarmac all day.
But as for JNB, there may be several reasons which include:
1. Late flights ex-JNB enable more connecting possibilities from domestic/regional points.
2. Less chance of running up against the European airport curfew.
3. Greater range of connecting possibilities after the flight reaches Europe (assuming it’s a morning arrival).