There were a number of favourable circumstances. I think the most important two were that 1) the editorial board had many who considered Open Access to be important, and 2) it was a relatively small field in which they managed to get their transition relatively well publicised, which allowed people to submit their articles there without negatively impacting their career.

Interestingly, I'm working on a blog post right now on what others can learn from this (because I'm trying to make it happen more often), so if you (or others reading this) are interested, you can follow along via https://tinyletter.com/Flockademic