Alphabet isn't the only big corp down after hours today.

As of now:

Microsoft - down 5%. http://www.marketwatch.com/investing/stock/msft

Visa - down 4% http://www.marketwatch.com/investing/stock/v

Starbux - down 4% http://www.marketwatch.com/investing/stock/sbux

In each case, the cause is different. But one thing is common, trailing PE ratios of all four are high compared to historical norms. High PE ratios are maintained with high growth in revenue, earnings and margins. Back off even a bit and the invisible hand smacks you in the face.