Assume that you aren't signed in Microsoft Skype for Business 2015 (Microsoft Lync 2013), and you try to join a meeting by clicking a meeting join link. When the sign-in process takes too long (for example, more than 10 seconds), Lync Web App will start.
How to get this update
To get this update, install the April 4, 2017, update (KB3178731) for Skype for Business 2015 (Lync 2013):
Note Lync 2013 was upgraded to Skype for Business in April 2015.