Unsubscribe reason states 'Reply Mail Management' when RMM is disabled for an account in the Marketing Cloud Email

These email clients are typically on a Unix based system. They can generate a response that uses the LEAVE option.
 

When the recipient uses this option, it causes the RMM process to pick up those replies and process the LIST_UNSUBSCRIBE header, which stores the LEAVE reply.
 

This type of reply uses the same process that the RMM process uses so it's recorded as such by the system. 
 

It isn't possible to differentiate between a client responding with the LEAVE command versus a standard RMM response.