Some people get so good at something that they become bored with it to a certain degree. They can't help but take their skill to the next level in an effort to continue to challenge themselves. For example, I have a friend who got so good at guitar that now he only plays it behind his head or with his teeth. That must have been the case with this slackliner who has gotten so good that now he's doing it upside down.
This type of slackliner picture is especially mesmerizing since he's upside down. The hanging quality gives it a topsy turvy feeling. It's like it totally feels like he shouldn't be in the same view as the water Try doing a headstand and looking at the picture and it'll look like he's doing it in bizarro world.