Continuity mistake: During the final basketball game, when Shep makes a finger roll for a layup, his jersey and 15 are backwards.
Continuity mistake: When Sheppard is playing against the birdmen, at one point he does a layup around a defensive player. In the following scenes, he does exactly the same layup from a reverse angle, and you can see that the defensive player's number is also backwards.
Answer: There is no in-film explanation for this nonsensical scene. It's a dilapidated hoop on a roof, so maybe we can assume there used to be safety netting there at some point, but it has since been removed or destroyed. The real-world answer is the filmmakers wanted the death to be a freak accident. It also had to be something Shep would be legally blamed for (the police thought Shep threw Nutso from the roof) and something that would make Shep feel immense guilt (he bet Nutso couldn't jump higher than he could). And of course, to make it topical to the plot of the film, it also had to occur during a basketball game. This is apparently the best they could come up with, and we are left with a classically ridiculous death scene that makes no sense at all.
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