Hi there! I work in the Google Open Source Programs Office. Echoing what others have said, it's usually just a matter of an engineer or team deciding it's something they want to do. Other times, it's a strategic choice.

We open sourced our open source policies/docs a while back, so if you're so inclined you can dig deeper there. These two links will be of particular interest: https://opensource.google.com/docs/creating/ and https://opensource.google.com/docs/why/