Actually they gave boosts to sites using HTTPS in rankings over a year ago and they are just expanding this. They also announce all this back on Wednesday, August 06, 2014. You can read about it on the Google Webmaster Blog: HTTPS as a ranking signal, or view the Google I/O 2014 – HTTPS Everywhere talk.
This is nothing new and not the reason for the move to HTTPS but it’s an indication that sites that do not update contain outdated information or are untrusted sources because of the lack of resources and updates. Updating to HTTPS is easy and free as you can see on this page.