Yahoo is likely to view your emails in order to better enable it to target ads to you. In the Wall Street Journal it has been found that yahoo's parent company Oath is in talks to pitch advertisers a service that basically analyses the contents of emails of about 200 million users.
However, users need not worry since their private messages won't be viewed. Users of yahoo can opt out of ad tracking which is possible through Yahoo's Ad Interest manager.
Yahoo is also not the first one to check users contents. Gmail when it was launched in the year 2004 had also appointed waves in order to view users contents. However it later announced that it will stop scanning user emails.