There may be a couple of things going on.
- If one or two rogue members start marking undesired incoming mail from your group as Spam because they don't/won't click Unsubscribe, Yahoo will send a notice to Groups.io that the messages are considered Spam.
- If there is a huge influx of mail from some other Groups.io group (like [GMF] following the Yahoo shutdown announcements), that could trigger a determination that everything from groups.io was Spam.
- As noted (see screenshot below), it's not hard to click Spam by mistake. When you click Delete in Yahoo webmail, you get an Undo popup for a few seconds. You don't get one if you click Spam. That should alert the user to find the item in the Spam folder and mark it as Not Spam.
Larry
Laurence Marks