I've tried launching three different MG casinos across three different PCs this evening, and they all immediately start background downloading.
Seems to be a small update to each game rather than one overarching update, certainly all the slots I've tried so far have a brief downloading period before launching, and this is despite every PC having a complete cache of all games in the C:\ProgramData\MGS\cache folder prior to today. (Has been stable at about 10.1GB for quite a while.)
I'm sure there's nothing sinister going on but one does wonder what the update is, I can understand a client software update, or new games being downloaded, but what needs changing on ancient games like Boogie Monsters? (Unless I'm mis-reading what's happening, but it seems odd that every individual game has a 'downloading' pause when launched.)