Valve is testing an option in the new Steam client beta that will force games by default to only download updates when you launch them.
Currently, Steam by default decides what game updates to download based on a few different things. As explained by Valve:
For games you recently played, Steam will download game updates shortly after they are released. For games that you haven’t played in a while, Steam might wait a few days to bundle multiple updates together or choose to download the…
