ProtonVPN was created in 2017 by a group of scientists from the European Organization for Nuclear Research (CERN), used to working with the community and convinced that joining forces to improve and review software development is truly beneficial for everyone. Therefore, they already had in mind to release the code of all their applications and now they have made it happen.
Regarding the decision to submit their technology to third-party review, in their blog they explain that they have done so to provide a higher level of transparency and responsibility to what is offered today in the market. From the beginning they established a no registration policy and are subject to the regulation of Switzerland, where some of the strictest privacy laws in the world govern. Now they have taken another step: Mozilla analyzes its technology and makes public all the results of the security audits.
For more information, download the code of all Proton vApp and audit reports, see this article at their official blog.