Leaders like Mugabe and dos Santos witnessed what happens to unpopular dictators like Saddam Hussein and Muammar Qaddafi when citizens rebel and the military is unwilling to protect them from the opposition. They have learned that (a) violent rebellion is possible even in the staunchest dictatorships, and (b) if your party (and your generals) no longer support you, you could be killed. Under these conditions, peaceful resignation suddenly seemed like an attractive option.