When you create multiple new endpoint contacts one after the another for a trusted application in Microsoft Skype for Business Server 2015, the endpoints will always be assigned to the same routing group against the same Front End Server. If the Front End Server goes down, all the endpoints will go into "reestablishing" state. Then, the Front End Server will be overloaded and meetings will be affected. It's expected that the endpoints that're created will be assigned to different routing groups to balance the load.
This issue occurs because the routing group IDs for trusted application endpoints don't populate correctly.
To fix this issue, install the January 2019 cumulative update 6.0.9319.537 for Skype for Business Server 2015, Core Components.