From the Outlook Web Access (OWA) interface for Nexus365 (Office365) the default setting for replying to messages is Reply all. This means that any message you reply to will be copied to all the email addresses mentioned in the TO and CC fields of the original message.
Some people believe that Reply all is a “dangerous” default for replying to email as you could inadvertently send information to people who you don’t know or who shouldn’t be receiving the information you are providing in the first place.
If you want to change how the Reply button works within Nexus365 OWA then you can change the setting from Reply all to just Reply; this will only send a message back to the original sender.
Office365 Reply Settings