Before he came to Apple, he was the heir-designate at Pepsi. It's bad execution. In about 70 per cent of cases, poor execution of strategies was the reason behind a CEO's downfall. There are other factors that might contribute to the CEO's fall - an antagonized board, or the CEO's espousal of flawed strategies. But these are secondary at best.
Execution is the key.
Most CEOs invite trouble by not putting the right people at the right places. Specifically, they do not deal with subordinates whose poor performance drags the entire company down. In that sense, the failure is one of emotional strength. Though these CEOs might be aware of such a problem, they rationalise it away. Even the issue of blind loyalty is more prevalent than what people think.