Remember last week, many many sites noticed their site was labeled with this site may be hacked warnings in Google Search? Google then told us it was potentially a misclassification of a new hack classifier they released. It seems to be fixed now, based on the sites I tracked as impacted by this.
Google now released a blog post saying that Chrome, Google's web browser will not just "showing warnings before you download unwanted software, Chrome will show you a new warning, like the one below, before you visit a site that encourages downloads of unwanted software."
Google took that to search as well, saying search "now incorporates signals that identify such deceptive sites." I suspect it was the site hack label and this new push but I don't know for sure.
Either way, this is a new hack classifier Google added to search, ads and Chrome that may result in more sites showing the hack label.
John Mueller from Google told me that this is unrelated to last week, he said "nope, these things are separate."
Forum discussion at Google+.