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/

+1

I can speak a little bit about what motivated us.

We saw from OSS-Fuzz (https://github.com/google/oss-fuzz) that this sort of thing could be widely useful and wanted non-open source code to benefit from making fuzzing easier.

Thanks Josh! Love the meta-open sourcing.

I would guess that it has to do with the usefulness of the project outside of Google. This project could be applied to so many other things (as OSS-Fuzz demonstrates), so open-sourcing it makes perfect sense. It isn’t some kind of classified algorithm, either.

Open sourcing is usually pushed from the bottom. People decide they care about open sourcing their project at they push for it.