In the book, Motorcycle Boy seems fascinated with the Rumble fish or Siamese fighting fish in Mr. Dobson's store. When Rusty asks him why each fish has to be kept in a separate fishbowl, Motorcycle Boy answers that the fish would kill each other if they were in the same bowl. He also maintains that the fish would kill themselves fighting their own reflection, if a mirror was propped up against the bowl.
After he says this, Motorcycle Boy asks a cryptic question. He wonders aloud if the fish would continue to fight if they were in the river. This rhetorical question foreshadows Motorcycle Boy's eventual actions in letting out all the animals in Mr. Dobson's store. As to why he does this, we can look to the text for some clues.
First, Rusty tells us that Mr. Dobson's store gave him the "creeps," with "all those little animals waiting around to belong to somebody." So, the plight of the caged animals symbolize a state of abandonment and longing, directly mirroring Motorcycle Boy's life in stark terms. In the story, Motorcycle Boy and Rusty were both abandoned by their mother in their childhood.
When Motorcycle Boy admits to Rusty that he had met up with their mother when he was in California, Rusty is incredulous. He wants to know how the meeting turned out. Motorcycle Boy tells Rusty that their mother was living with a movie producer and that she eventually planned to move in with an artist up in the mountains. From the story, we realize that their mother has moved on with her life and is seemingly oblivious to the turmoil and pain she had caused her sons when she abandoned them in their childhood. By all indications, Motorcycle Boy has found no comfort in meeting up with her.
He finds himself trapped in the past, with questions a self-absorbed mother cannot answer. So, his actions in freeing the animals are largely symbolic. He wants to free them because they are helpless and seemingly abandoned, just like him. He even takes the fish out because he wants to put them into the river, where he imagines that they will no longer have to fight. Perhaps Motorcycle Boy's supposed ignorance of the police officer's warning shot is intentional. He too, wants to be free from rejection, emotional pain, and conflict. The text tells us that Motorcycle Boy died with a smile on his face, perhaps because he realizes that in death, he will be as free as the animals he has liberated.