The reasons, well it's a real smorgasbord of all the usual suspects.
There is no clear evidence for the reason for this apparent decrease. But it has been linked with exposure to chemicals used in pesticides and plastics, obesity, smoking, stress, diet, and even watching too much TV.
To be fair the article does mention there have been criticisms of these studies, mentioning publication bias unusually amongst other things. But most people's takeaway will be we're doomed.
I think this probably marks the official start of the silly season. If this standard is kept up it could be a classic.
The Levine paper and also the Washington Post news article mentioned in the Milloy blog post don't mention the 'possible extinction of the human race' idea - that seems to be a piece of sensationalism that is coming from the BBC.