Beehaw's purpose is to have a place to have certain discussions in an environment that is conducive to those discussions. To build that environment, they have a user application process for new users. That was working well for them. However, lemmy.world allows anyone to sign up without screening. The result was that people were able to bypass Beehaw's screening process to damage their environment.

To put into perspective, imagine you're trying to run a support group for civilian refugees of Ukraine. You create an application process to screen out anyone that would like to sabotage that environment. However, the room also has a window to an adjacent room, and that adjacent room is open to the public. Russian military then use access to that room to taunt the people attending the support group with pictures of dead Ukrainians, not only effectively ruining the purpose of the group, but making the group members' lives worse.

What Beehaw did was concrete up the window until they can figure out how to better protect their group members from people that want to damage their environment.