I think it became the center of the protest simply because it was the most popular Reddit iOS app and the developer actively spoke out against accusations from the Reddit CEO.

In terms of why it was so popular I can only tell you my own experience which started with bacon reader which like most Reddit apps at the time mostly mimicked the old reddit site. Then I switched to alien blue which used more of the iOS best practices such as larger buttons, swipes, etc. When alien blue turned into the official Reddit app, it became bloated with ads, suggested posts and other annoyances. At which point I switched to apollo which had the similar nice UI without the baggage of the official app.

But honestly I think I only got off the official app because of something annoying with collapsing threads that I have since forgotten.

I created an account and nothing loaded but the FAQs suggests an interesting concept. It’s almost like a wiki Reddit where people should be able to correct and edit other posts and comments. Not sure how it would work practically but I would be very curious to see what happens