Hi Hellios,
OK, I misunderstood your post somewhat then
.
South Africa was ruled by an apartheid regime at the time, but they didn't invade Angola as such, in that they never intended to occupy Angola but to provide support to one of the rival factions in the war (UNITA).
It just seemed from you're post that MPLA defeated forces which were pro-apartheid when you should actually say that the the MPLA coalition (Soviet Union, Cuba, MPLA) defeated the UNITA coalition which included Apartheid South African soldiers.
South Africa never supported UNITA because UNITA might be supportive of South Africa's apartheid policy but because they fought communist forces. As Kapikulu stated, cold war politics. The South African government tapped into the cold war politics to create the fear amongst white South Africans that a change to majority (black) rule would mean a change to communism for the country. They therefore had to be seen as fighting against the communist threat that was seemingly surrounding South Africa.