With last week’s tech stock wobble, investors looked over the abyss for the third time in less than two months. After last year’s unnatural calm, we can’t seem to find enough ways to test our faith in the nine-year bull market. First, we worried about inflation and interest rates – that was Fed chairman Jay Powell’s baptism of fire.

Next, we focused on growth – fretting about Donald Trump’s America First protectionism. Now, it’s technology’s turn, as the sector which has largely driven the post-crisis rally comes under the spotlight.

There have been three apparent causes of the recent stumble of the FANGs (Facebook, Amazon, Netflix and Google). They are all contributors but none is the real story,...