If the publisher of a game only signs the contract to sell the game for a limited time, or if the license for part of the game is time-limited, or is the publisher simply goes bankrupt, a game can end up in a spot where no one entity owns the rights to the whole game. In these cases, the game typically goes out of print and is taken down from digital download platforms such as Steam. Games taken down from Steam in this fashion are in all known cases thus far still on the platform to re-download for those who had purchased it before getting taken down.
This is a unique problem that digital distribution faces. If a publisher looses a license for a boxed retail title, for instance, the game can still be sold second hand, thus it's still aquireable. Digital downloads however can not be sold used and so the gamer is at the mercy of waiting until another publisher picks up the rights to the game again.
This does not mean that all games taken down from Steam qualify for the concept: a game might be taken down because an updated re-release is released, or simply because the publisher no longer supports the platform for certain games, as in EA Games taking Dragon Age 2 and Crysis 2 down from Steam with the launch of EA's own distribution platform, Origin.