I find this happens most often to my users when they are trying to reconnect to a RemoteApp that they had left open and got disconnected from. I've seen it happen to Word & Excel, as well as a number of internal applications that we use here.
Unfortunately, like you, I usually have to log in to the server and boot them off.... I usually just keep an RDS session open to that server and leave a script running to make it easier for me to kill a user's hung session.
https://gitlab.wellston.biz/dafyre/powershell-scripts
autoUserSessionDetails.ps1 Should help you with that. If your machine is AD joined, you can run it from your own machine, but I still recommend running it from the Connection Broker.