In the September 2015 update of "Outlook on the web", users have noticed that the default reply setting has changed from "reply" to "reply all". This could cause users to inadvertently reply to all users in an email thread when they intended to reply to the sender.
If you click on the button, the default action is to perform a "reply all".
To reply or forward the message, you must click on the down arrow next to this action and select the desired action:
To change Outlook on the web's default setting back to "Reply":