Basically whoever has the most seniority has complete control over the subreddit, they can remove other mods or override any lower-ranked mods decisions. That doesn't mean they would, and I trust the new top mod, but it also puts a bit of a target on their back (top mods get blamed for a lot), means they're going to be dealing with a lot of people private-messaging them instead of messaging the subreddit, etc.

Basically "ownership" of the subreddit has changed hands after 10 years to the person with second highest seniority (at 5 years).