How did the joker return in arkham city

In the end of arkham asylum, joker was defeated in his titan form and later returned to normal. He was still in arkham asylum when you fought, so he just had to be captured by police. Harley Quinn escaped the asylum's police and broke out the joker when he purposely got solitary confinement by digging a guy's eyes out with a spoon. They eventually had to swim to the shores of Gotham, but they found that the half they were on was changed to arkham city.