Outlook also recently changed the default “report message” action in the UI to be “report phishing/malware” instead of “report spam”. This was a terrible design choice; phishing reports from my org’s own user base has increased 4x since the change which is a lot of false positives.

So maybe folks mean to “report spam” on your emails but “report phishing” instead…

Could be the reason, but even so, we have really low volume campaigns and mostly to people we actually interact / have a history with. I would assume it would take more than a few accidental hits to trigger this issue.