This is one of those things that makes me wish there was a way that subscribers of a subreddit could have some say regarding moderators - nominating people to be mods, perhaps voting to remove a mod that has become abusive or absent, etc. I do see a huge potential for that to be manipulated in some really bad ways, but I still can't help but think there is a better system to be had.

On fetlife.com (very NSFW), anyone can volunteer to become a moderator if the group becomes moderator-less. Hell, that's how I became the moderator of the reddit group on fetlife.