They can be as hostile as they want; that seems nearly irrelevant to the fork decision. If the mainline won’t take a patch or wants to go in a different direction, forking seems perfectly valid and they can keep their empire. That seems fine; they didn’t want to go east, the fork going east means that those users who also want to go east can be served.
This isn’t necessary. Maybe not even common. Forks can start as a testing ground or an experimental feature fork and grow from there.
We see headlines about the angry forks, but usually it’s just friendly differences.
The problems arise when one person wants to control the project, deny contributions, but also gets angry when someone forks the project to implement those things. They put the open source license on the repo but didn’t expect other people to actually do open source things with it.