During Fast User Switching, Desktop Window Manager (DWM) has to restart for user sessions that log on, and it has to shut down for user sessions that log off. Because there is a DWM transition occurring, flickering also occurs. This flicker does not signify that DWM is crashing, but rather is part of the Fast User Switching transition that requires DWM to restart. This behavior (restarting DWM and therefore causing the flicker) is by design.
We will consider rearchitecting DWM to avoid the flicker in a future version of the Windows operating system.
Note This might be reproducible on Windows Server 2012 R2 when you use Fast User Switching while multiple users are logged on.