My informant heard this from her Vietnamese grandmother when she was a child.
“When a fruit bruises, it’s because a ghost bites it.”
She thinks it’s weird and funny. She likes the image of a ghost trying to bite a fruit.
Ghosts are much more natural and interact more with the world in Asian cultures than in western ones. Also when a fruit bruises it means its gone bad or damaged which ties in with death. It is as if the ghost cannot fully interact with the world and leaves destruction behind instead. It can’t quite bite through the fruit but it is enough to leave a mark. Fruit is also left as an offering at graves, so it shows that the spirit of the dead person the offering was left for is enjoying the gift.