Comment by hawaiianbrah

> When Mark’s and Cassio’s photos were automatically uploaded from their phones to Google’s servers, this technology flagged them. Jon Callas of the E.F.F. called the scanning intrusive, saying a family photo album on someone’s personal device should be a “private sphere.” (A Google spokeswoman said the company scans only when an “affirmative action” is taken by a user; that includes when the user’s phone backs up photos to the company’s cloud.)

I assume it was triggered when the photos were backed up to Google Photos based on the above quote.