They defederated because they were both large Lemmy instances with zero review process for joining users, and they’d rapidly starting acquiring bots and bad actors. Because of federation, these accounts could interact on Beehaw’s server like they were locals.
Beehaw on the other hand, has a human-powered review process for signup. It isn’t strict, but it keeps out bots or low-effort users. Beehaw’s community goal means that reducing the amount of bots, bad actors, and low-effort users on the platform is a priority for them. Their moderating is also human-powered, and very involved - not outright banning/blocking. They reach out to users to discuss their content’s intent, and issue warnings/requests personally as needed.
That level of moderation is fantastic for fostering community and is compassionate for ignorance and error; but it isn’t scalable when being hammered by bots and an influx of new accounts. Beehaw’s only protection from instances that shelter bots and bad actors was to defederate from them until those instances were able to address them somehow.
The Beehaw admins have reached out to the admins of the other instances; their hope is to find a solution that reduces the amount of bots and spam accounts creating on .world and .works. They don’t want defederation to be a permanent solution, it’s just the only feasible one they had.