starting on page 141 with Darl narrating, why does he jump off the wagon, and what does this foreshadow for him?

He jumps off of the wagon because they're crossing the river, and a log rushes them at the ford and unsettles everything. Jewel tries to keep gold of the horse, Cash tries to hold onto his tools and the coffin, Darl jumps, and the mules drown.

"When Darl describes the desolate air that surrounds the wagon as it enters the river, which he compares to “the place where the motion of the wasted world accelerates just before the final precipice,” he employs particularly fatalistic language. Cast in this light, the river becomes a final frontier separating the Bundrens from the next life, and given the circumstances that lead up to this journey, it is hard to gauge whether Addie is being sent off to heaven or to hell."