For example the current controversial change: Google says "slow web is bad" and deprecates an API that allowed extensions to do almost unlimited work for any web page. Oops. Adblockers depended on that and did want to do almost unlimited work for any web page, and that wasn't a bug, but the API gets dropped, AIUI because Google wants performance. The reason doesn't matter though. Google gets to set the agenda, no matter what its reasons are.
[0]https://twitter.com/flamsmark/status/1088170219695071232?s=0...
a) ongoing cost of merging
b) security updates delayed because of merging.
This means that there is a limit to how much any fork can allow itself to diverge. They will have to pick their battles and accept the least-worst.It's possible the limit is high enough that it wont be a problem in practice though.