I also noticed this trend, but I don't understand the reasons, the site also doesn't explain it. Wouldn't it be beneficial to capture (and potentially track) existing users earlier? What's good in annoying them? Why are dominant sign in/register buttons mutually exclusive? What became of the trend of the reverse - large sign-in with a later option to register?
My cynical guess is that when companies A/B test this, they find that a clear sign-in button reduces the number of new sign ups (because if the sign-in button is hard to find, some existing users will create a new account). A/B testing is great but it's very easy to optimize for something that's easy to measure rather than what you fundamentally care about.