This may be the issue, though the resolution doesn't work for everyone:
MEDIAPORTAL - free MediaCenter HTPC Software - MediaPortal gets minimized to tray when playback of an avi is started
I didn't find this worked, other option is to change to EVR renderer in MP config or disable 2nd tickbox option under VRM9 (something about keeping MP on top) - this one may cause tearing, but i didn't find any tearing - so all good for me.
See this https://forum.team-mediaportal.com/general-support-51/mp-randomly-minimizes-video-playback-43658/
MEDIAPORTAL - free MediaCenter HTPC Software - MediaPortal gets minimized to tray when playback of an avi is started
I didn't find this worked, other option is to change to EVR renderer in MP config or disable 2nd tickbox option under VRM9 (something about keeping MP on top) - this one may cause tearing, but i didn't find any tearing - so all good for me.
See this https://forum.team-mediaportal.com/general-support-51/mp-randomly-minimizes-video-playback-43658/