we'll never know, Some say i'ts normal biological evolution, others say it's the CIA (or other such organisation) that was experimenting on biological warfare that went out of control.
I read a story a couple of years ago ( in the Times I think ) , that suggested that a doctor may have introduced HIV to humans while preparing vaccines for an unrelated disease. The vaccines were grown on Monkey livers - which could have caused the virus to cross species. The mass vaccination seems to corrspond to the area and period when HIV first started to appear in humans.