The Fine Brothers trademarked "reaction", "_____ reacts to", and various other series they have done, and the Internet interprets it as them trademarking reaction videos in general which is too big of a genre for one entity to own. The public isn't that off though, as the Fine Brothers have been known to take down various reaction videos, like a Ellen DeGeneres kids react to typewriters clip. Thus, they have been steadily losing subscribers. Last week they had ~14 million, now they have lost 100k subscribers and are at ~13.9 million.
The video announcing their trademark, portraying it as something for the community or something
Satirical take on this
Live subscriber count: https://akshatmittal.../#!/TheFineBros