Google posted on their webmaster channel on Google+ that one of the primary reasons a web site is hacked these days is because of SEO. Google wrote, "search Engine Optimization 'SEO' spam is one of the primary reasons that vulnerable websites are targeted by hackers."
I assume this is not just about redirecting unsuspecting users, but also link injection, content injection and many more ways to hack and boost your search rankings.
Search Engine Optimization 'SEO' spam is one of the primary reasons that vulnerable websites are targeted by hackers. Some forms of SEO spam can redirect your users to malicious sites and cause you to lose customers.
An early warning sign of this can be from customers trying to access your site. Be sure to pay attention to user feedback, questions, or complaints. Also, sign up for Search Console. If Google detects any malicious redirects, we'll send you a message in Search Console. #NoHacked
Learn more about hacking trends in Sucuri's Hacked Website Report: https://goo.gl/xlH1MI.
This doesn't make all SEOs bad, it just makes the hackers bad - I hope.
Forum discussion at Google+.