Moderators(who are doing tons of free work to make reddit a tolerable place) use API based tools to help them moderate. If reddit is going to make their (volunteer!) job harder, why shouldn't they just pack up and leave?
The official reddit app reportedly does not make adequate accommodations for some disabled users, but dedicated third party developers did. They later said that they would give exemptions for accessibility use cases, but stipulated that they could not charge for their service. Some people the perceived this as: "we won't make our app accessible, we'll give permission to do free labor for us if you want, but don't you dare try to get paid for it."
Then there is the disingenuous interactions with the Apollo developer.
All-in-all, people think that reddit is acting in bad faith.