Everyone here points to the high P/E ratio as the reason to sell. That is simply the reason to sell NOW. The reason it's being sold is he is obviously less bullish on the company. It doesn't take much to understand why. The thing that led to Apple's growth as a company and a stock is something called the iPhone. The thing that drove sales of the iPhone was the fact they were not just getting better, they were getting _different_. More cameras. Different kinds of sensors. Now they are just getting better. And less better each generation. Benedict Evans would call this the top of the "S" curve. It matters when selling someone a new phone in a new box is the main thing that generates your profits.
You can be a great company with great leadership and great products. You can't defy the laws of the "S" curve and market saturation. It doesn't matter how good you are. This is why Buffett and Co. are unloading. I wouldn't be surprised if it all gets sold by the end of the year.
EDIT: I called this sale two weeks ago in a tweet: