From what I saw the lower mod probably contacted the admins to make this happen by claiming that the inactive head mod went against the wishes of the community.

It's happened before to other subs like /r/wow but back then it was a positive move to circumvent a man child making the sub private over an expansion release that didn't go so well because he couldn't log in and play the game. A lesser mod was made head and the original head striped of his position and all was made right again.

In this case it appears that the community overwhelmingly supported the blackout and move to go private instead. The mod that took over was the only loud voice against the blackout.

The Reddit admins indicated in a modsupport thread that they would help any mods who didn't want their sub to go dark to take over from other mods.