RDP Sessions Disconnected

Hello,
More of a general question than a complete issue. One of our users used Remote Desktop Manager to RDP into a few Server 2012 systems from a remote VM. He left the connections open and closed his connection to the remote VM (His account was still signed in and running Remote Desktop Manager). On Monday when he logged into this VM he noticed that the Remote Desktop Manager console was trying to reconnect to one of the RDP sessions, it ran to attempt #4 and then both of the embedded tabs closed, it appears from the logs that one of the sessions ended on Sunday early morning and the other one must have stayed running (No end time). From my understanding Remote Desktop Manager should have kept the connections open all weekend and when he logged back into the remote VM the RDP connections should have just been locked. Any explanation as to why the connections disconnected? Your help is much appreciated!