They tried to apply a per install fee and make it retroactive for all games using the Unity Runtime. Meaning games made with Unity even a few years ago who didn’t agree to this would be charged each time someone uninstalls and reinstalls the game.
This is the second time they have tried to make retroactive changes, by the way, and they even deleted the part of the TOS that prevented retroactive changes.
It’s really just one of many things they have done in the last few years