Cats go missing all the time. Heck, mine disappears about 12 times a day and that's within the confinements of the apartment!
Another obvious thing cats really enjoy is climbing.
Climbing. Literally. Anything. If it's climbable, your cat will be all over it. If it's not climbable, your cat will be all over it.
It's just what they do. What you rarely see, however, is a cat holding onto a tree branch... Cats are usually such elegant creatures. It's quite silly to see them in a position that showcases the opposite of that.
Don't worry too much, though, the owner has confirmed below that the cat landed safely and (if you can believe it) elegantly.