Citing a report by the Cyber Threat Alliance, the spike is tied to the 2017 leak of Eternal Blue, the NSA hacking exploit that utilizes a flaw in Microsoft operating systems to allow hackers unprecedented access to otherwise secure cyber-infrastructure. The exploit first made headlines last summer, when it was publicly leaked by the Shadow Brokers, a mysterious group of hackers who allegedly have ties to Russia or North Korea (who can keep track?).
The Shadow Brokers famously offered a subscription service to hacking groups allowing them preferential access to a trove of NSA hacking tools that the group allegedly stole from the US government. After the Eternal Blue exploit was linked to both the WannaCry and NotPetya global hacking attacks, the group's public profile diminished significantly, but continued to release tools allegedly stolen in that breach.