When a player leaves or is kicked out of a guild, if enough GP is removed from buildings, one or more buildings may drop down in level. At that point the benefits from those buildings to guild members drop to the lower level until more GP is added to level it back up.
I have not played Splinterlands in the recent past, but this addition sounds fair and obvious. I wish that I could say the same about the entire Splinterlands game. Once it was one of my favorite games. But nowadays it is too hard (frustatingly hard), and it is rather investment based than skill based. It is rather a pay-to-win game. This is why I do not play Splinterlands nowadays.