During the summer session, jumping out a the tree into the river had been an act of rebellion against the rules and a demonstration of the carefree attitude held by the members of the Super Suicide Society of the Summer Session. After Finny fell from the tree, with the push from Gene that he still couldn't explain but which caused feelings of tremendous guilt, Gene wanted to apply Dr. Stanpole's verdict that "Sports are finished" to himself as well as to Finny.
Gene was afraid of what he had done and was afraid of what he saw the others doing in other sports. The peaceful atmosphere of the summer's activities was gone, in sports as in other aspects of life at Devon.
It was as though football players were really bent on crushing the life out of each other, as though boxers were in combat to the death, as though even a tennis ball might turn into a bullet.
In the winter session, jumping into water from a tree
stood for abandoning a torpedoed ship...after you hit the water you made big splashes with your hands, to scatter the flaming oil which would be on the surface
Gene wanted to distance himself as far as possible from acts that held significance in the personal war he felt raging within himself. He had committed an act of aggression against Finny and wanted nothing more to do with any of it. He wanted to spend his required involvement with sports doing the most passive, unagressive and uninvolved activities he could find, such as acting as assistant senior crew manager.