For details: All v3 Onion Addresses Down After Attack On The Tor Network (darknetdaily.com)
The Tor network is not fully decentralized. When you first connect to the Tor network there is hard coded IPs that your Tor process uses to bootstrap your connection into the Tor network. These IPs allow your Tor process to load up the network’s consensus. This consensus tells the Tor process things like what relays are within the network, which are good relays, bad relays, which are guards, exit nodes, how much traffic a relay can handle, that kind of idea. Your Tor process gets all that information and validates it by signatures of these hard coded IPs. These hard coded IPs are called authority nodes. There is currently 10 of them on the Tor network. And they are why the Tor network cleared out V3 onions for a period of time.
The authority nodes “vote” on a majority consensus they all share with the Tor network. Generally a new vote happens every hour and the voting process takes 5 minutes. If there is no consensus for three times in a row (as in for three hours) the health the network goes massively down. You can check consensus health at this URL https://consensus-health.torproject.org/. The vote decides a lot of things in the network and when the consensus can’t be succeeded, there is a lot of issues that can occur. Things like V3 Directory variables not being included within a valid consensus so all V3 onions become unreachable.
The attack basically overloads the authority nodes by sucking up all their bandwidth so the authority nodes can’t communicate between themselves to vote and make a consensus. This fundamentally breaks the network if it goes on too long. This isn’t so new. Like a lot of the Tor attack issues which get exploited in this way there is a closed issue on it (gitlab.torproject.org).
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