Facebook started notifying members of a group that a certain top post has been liked. Well, Facebook always has a knack in irritating people off with their creativity. Luckily, this is one of those where it can be fixed.
On your group, click the cover photo and it should bring you to the settings page.
On the settings page, click 'edit notifications setting'
On the notifications settings, make sure anything to do with 'highlights' is not selected.