Sucuri just published an interesting article that shows Google is starting to slowly force SSL on all websites. The gist of it is: websites can now get blacklisted just for not using SSL. The affected websites appear to use non password input fields, or use another type of login. For example, the WordPress admin login counts as a login as well.
This seems quite a harsh measure for sites that are clean, without any external resources, and seems a sign that Google is now pushing hard for an all https internet.
There seems to be only one solution of your site got blacklisted for no apparent reason: migrate to SSL!
In addition to the ranking benefit Google already is giving out, it seems there are less and less arguments to keep your site on http.
You can read the entire article here: