Gmail has a system of reputation as you suggest. It is very likely that enough people marked their emails as spam, which the OOP could figure out on the postmaster dashboard if they were so inclined: https://postmaster.google.com/managedomains

It also goes the other way, if enough people click "not spam" and interact with your mails, your reputation gets better. I'm currently trying to do that with my personal email/domain - will probably take some time though. For now, my friends say my mails land in spam even though I get a 10/10 score on mail-tester.com and similar sites.