A contributor license agreement, or CLA, usually (but not always) includes an important clause: a copyright assignment.
This is a strategy employed by commercial companies with one purpose only: to place a rug under the project, so that they can pull at the first sign of a bad quarter. This strategy exists to subvert the open source social contract. These companies wish to enjoy the market appeal of open source and the free labor of their community to improve their product, but do not want to secure these contributors any rights over their work.