The group was responsible for the attack on Colonial Pipeline last Spring, which used encryption software named "DarkSide."
According to Reuters, the US Government amped up its attempts to disrupt the group after they compromised Kaseya over the summer. It appears that law enforcement had successfully gotten into REvil's infrastructure earlier this year, but the group had shut down everything right after the Kaseya hack.
When one member of the group restored their network from a backup, they inadvertently also brought back up the servers which the government was already in, allowing them to further investigate and silently attack.
According to those Reuters interviewed, this is just one part of a massive effort the U.S. Government is carrying out to cut off cybercrime while also adding heavy defenses to their own network.