If you get more than a tiny handful of these, shut the account down. Whatever the reason, users don't want the messages and it'll destroy your deliverability quickly. Assuming the sender is legit, they need to figure out why users don't like the messages.
However, the much more common case is that a recipient's mail provider categorizes a message as spam on its own, without any user involvement. That's what parent commenters are referring to.
That can be for any reason. Maybe the user has reported similar past messages as spam. OTOH, maybe other users at the recipient service have had low engagement with messages from the same sender. Or maybe there was a sudden increase in sending volume from a given netblock or domain.
Emails that the receiving system categorizes as spam do not trigger any notification to the sender. Other than by trying to deliver messages to canary accounts or waiting for customer complaints, you won't know this is happening.
https://docs.aws.amazon.com/ses/latest/dg/monitor-sending-ac...