Microsoft is always striving to improve and streamline your product experiences—offering a new way to use the classic Microsoft Outlook app on Windows and the Microsoft Edge web browser.
Browser links from the Outlook app will open in Microsoft Edge by default, right alongside the email they’re from in the Microsoft Edge sidebar pane. This allows you to easily access, read, and respond to the message using your matching authenticated profile. No more disruptive switching—just your email and the web content you need to reference, in a single, side-by-side view.
If you would like to disable this, you can by, clicking on the 'Manage my settings' 
You then select 'Use system default' to go back to Chome.
Choosing which app to open hyperlinks in Outlook Classic
File > Options > Advanced > Link handling in Outlook and select a different browser
