And an unfollow on twitter really means nothing, lets not say "I will miss them together"... until he publicly says he doesn't want to work with her then say that.
Porcelain Black unfollowed Gaga too, so it probably had something to do with her. Gaga was phasing him out anyways, which she had to do. If she stayed with him her whole career everyone would label RedOne the brains behind the success anyways.