Question

why are the young trees described as sprinting

Answer 1

personified in the poem. They seem to be running in the opposite direction when seen through the window of the moving car. The movement is juxtaposed with the expression on the mother's face i.e. ashen like a corpse.

Why are the young trees described as 

Answer 2

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It is because when the car was moving hence the trees appeared to be running in the opposite direction.