What is the environmental pressure? Just a mutation by itself doesn't spread, there should be some advantage? Shouldn't there?
They seem to be suggesting that this is not necessarily an advantage. Modern medicine makes pregnancies with these genetic mutations more viable, so the dominance of the gene expression becomes a more significant factor than fitness.
There are a lot of potential causes. In my estimation the two most likely causes—
First, hitchhiking. The mutation could be near some other beneficial mutation. The closer genes together on a chromosome the less frequently they'll be separated by crossing over events.
Second, it could have some other, presently unobserved cardiac effect that reduces infant or maternal mortality.
We are living our whole life at a desk, it is providing extra endurance for clicking.