What is the climax of Hoops and what is so ironic about the ending of the novel? Why did the author choose to end it that way?

The book builds up to the point where Cal has to choose whether or not to put Lonnie in the game. If he does, he breaks his word and angers the members of a gang he made a deal with. If he doesn’t, he severely damages Lonnie’s chances of becoming a pro basketball player. Eventually, Cal decides to put Lonnie in the game, and as a result of breaking his word, Cal is killed.
Here’s the irony: Cal warned Lonnie against selling out earlier in the story. In fact, Cal had his own shot at playing pro basketball before he started coaching, and he blew it. When he had to decide whether to put Lonnie in the game, it was like he'd been given a second chance—and he chose to redeem himself by allowing Lonnie to play. This is ironic because instead of having positive results for Cal, it caused his death.
As for the reasons behind the ending, my opinion is that Myers wanted it to have a strong impact on the reader. Below is a link to an interview with Myers about another of his novels (Game). One of the first things he says is that “in life . . . things are not usually fair," which ties in with the unfairness of what happened to Cal in light of his decision to put Lonnie in the game. The interview also details Myers’s thoughts on how racism and poverty can limit the opportunities people have. This is evident in the setting of Hoops and the situation Cal finds himself in with Tyrone—and in Cal and Lonnie's lives in general. Overall, the ending is reflective of Myers’s ideas about the connections between poverty, racism, and fairness.
https://mosaicmagazine.org/walter-dean-myers-interview/