Now that the Gotham War is over between Batman and Catwoman, leaving Catwoman presumed dead, her plans to reform Gotham's low level crooks ruined and Batman on the outs from the rest of the Bat-Family, it is the perfect time for the Joker to stick his annoying laughing head into the mix, and he does so in Batman #139.
Source:https://www.cbr.com/batman-gcpd-joker-related-crimes/