Why did Maniac leave Russell on the railroad trestle?

This question is asking about an event that happens in chapter 44; however, the answer is found in chapter 45. At the end of chapter 44, Piper runs up to Maniac and Mars Bar and begs them to come help. Piper and Russell had been playing "Bombs Away," but their game took a turn for the worse when Russell realized exactly how high above the river he really was. He's up there frozen in fear.

And that's where Russell was now, out on the middle of the trestle, high over the water, frozen in terror, not even a railing to cling to, responding neither to Piper's cries nor to the red-and-yellow P & W trolley, which also occupied the trestle, idling and tooting about twenty feet away.

Piper is hoping that fearless Maniac Magee can rescue Russell; however, Maniac turns and walks away.
Chapter 45 begins with Mars Bar seeking out Maniac. Mars Bar wants to know why Maniac didn't help Russell. Maniac isn't keen to talk about it, but he eventually explains why he couldn't help Russell. Maniac was just as frozen in fear as Russell. That railroad trestle is the same trestle where his parents were killed in the train accident. Maniac can't stand the sight of it, and being up near Russell caused Maniac to envision what it must have been like for his parents as they plummeted to their deaths.

Maniac told him the story of his parents' death. He told about his problem with the trestle, how he had learned to avoid it. "And then, all of a sudden, there I was, on the platform, looking out at it, closer to it than I ever was before, up on the same level. I always saw it from below before. Now I was up there, too, where they were, looking down, and it was more real than ever. The nightmare was worse than ever. I saw the trolley coming ... I saw it...f-falling...them...them..."