Several things have changed over the last two years that allowed us to make this happen: Most importantly, we've continued building out the product for a more broad user base, grown with more customers and users, received further funding as validation, and expanded both our team and product to offer additional revenue generating services. All put together, open sourcing Citus is something we've always wanted to do, and we are excited to continue building on it for many years to come, with the help of the community and our enterprise customers.
Beyond that, we have a few other things in the works for the future that will cover other revenue models.
Companies of any appreciable size will be happy to pay for support if they choose to make Citus a part of their critical infrastructure. And the industry reached an inflection point where there are enough companies want as much of their infrastructure to be open source as possible, that you can run a company where most of your stuff is open source, while still making a ton of money (like RedHat, CoreOS, Docker, etc)
But a sign of getting investors is also that they see that there's still potential of making a lot of money, in spite of being open source.
That's true here too. Citus mentioned they spoke to their board (and presumably their investors too) about this change.
AGPL means the only people using it have to licensed the same.
AGPL only requires open sourcing any modifications you make to the software when you give users direct access to a server running the software, which seems like something you would never want to do in case of a database.
You can use database servers running AGPL software in a closed source SaaS: http://www.gnu.org/licenses/why-affero-gpl.en.html