It's been more prevalent recently and I think it's just part of a larger fear of guilt by association. Some other comment mentioned banks/payment gateways being a part of it, but I don't think it's just that.

A more reputable company just might not want their ad displayed on a site that might have porn or questionable content. Look at how YouTube ads and monitization have changed in just the past few years. There were real issues with major ads getting displayed on popular but questionable videos.

We're heading into a much bigger issue of call-out culture. We're seeing academics, programmers, CEOs, authors and comedians being told they need to resign from positions or be ousted for single political comments or a single donation to one cause made years ago. Then people go back through history and pull up every questionable thing that person has done or said.

I don't think it's tangentially related; and that it's affecting the business of Internet advertising.