It's semi-voluntary; the request to open source the application came from the Dutch congress/2nd chamber if I recall, but took a while due to private information leaking concerns.
It was released as the result of a Freedom of Information (WOO/WOB) request made by serial "WOBBER/WOOOER" @BugBlauw, check his twitter (use google translate, works well with Dutch).
https://twitter.com/bugblauw