https://blog.torproject.org/tor-network-ddos-attack/

For at least 7 months, several different types of ongoing denial of service (DoS) attacks have affected the Tor network. At some points, the attacks impacted the network severely enough that users could not load pages or access onion services.

We have been working hard to mitigate the impacts and defend the network from these attacks. The methods and targets of these attacks have changed over time and we are adapting as these attacks continue. It's not possible to determine with certainty who is conducting these attacks or their intentions.

We will continue to increase and tweak defenses on the Tor network to combat this problem. We are also adding two new members to our Network team who will be solely focused on .onion services development.