You used to get these sequels for games, which later on turned into expansions and then developed into this DLC thing. These days atleast the major companies plan a lot ahead. They tell the developers what to do in advance. Developers accept the terms because they need money to make the games. In these contracts, the developer is required to do updates to the game for period of time or even a sequel - or DLC, which is propably more cost free to do.
If the developer is owned by larger company, then they even might nominate a pointman to the project (game) that get's developed and say what there is going to be and so on. Good example of this is the Mafia II game where the 2009 pre-material shows missions that are not in the final game but instead, sold as DLC "jimmy's vendetta". The DLC lasts as long as the maingame but is sold at 10 bucks. I guess the 2K got a heart now since selling it as full game with 50$ pricetag would have been a ripoff - or perhaps Mafia II was as the content was cut due to "plot not working in-game".