Like any commodity, the value of the rand is determined by the market forces of supply and demand. Its weakening is also affected by a myriad of structural problems facing the South African economy.
South Africa has had three finance ministers in four days. President Jacob Zuma will live with the fall-out for the rest of his term. Markets have a long-term memory and won't easily forget.
The removal of Minister Nene is a significant act. The fact that he has been replaced by someone who is unknown and untested simply compounds it.