A cager in basketball is a reference to a period of time where basketball games were played in enclosed cages rather than the open-air stands we see today. Cages were used to hold fans back and can be used as a reference to streetball settings today. Cages were considered to make the game faster and more entertaining as when a ball went out of bounds the first player to get the ball could throw it back in instead of the rules we have in place today.
Back when basketball was played in a cage, players earned the nickname cagers. This is because they not only played in a cage but it was also a useful shorthand for newspaper headline writers covering the sport. It is much easier to write "cager" than "basketball player" in a headline. Similarly, the game overall was called the "cage game" as it was easier to use in headlines.
The game of basketball with cages became less and less after 1929 as teams in other leagues had begun to use no cage to good results. Some leagues continued to use cages into the 1930s merely because of tradition. However, it was obvious the sport was going in a different direction.