In A Separate Peace, what is Leper's description of the accident, and why is his story so upsetting to Finny?

These answers can be found on the last two pages of chapter 11, if you are interested in finding the actual quotes.  Leper actually describes the event very poetically, when he is assembled before the "council" that Brinker has gathered together.  Leper starts off by describing Gene and Finny's positions on the tree.  He states that "one of them was next to the trunk, holding" on to the tree, and the other one was "a little farther out on the limb."  Then, Leper uses a simile to describe what happens next.  He said that they both moved "like an engine" and went on to clarify that he is speaking of the piston of an engine and how "one piston sinks, and then the next one sinks", indicating that whoever was close to the trunk, holding on, caused the limb to sink down, which knocked the other one off balance, causing him to fall off.  When Brinker asks him to clarify who was the one that was next to the tree, Leper refuses, but Finny knows, and has a pretty violent reaction.  For one of the only times in the book, Finny curses, and is really angry.  He curses at Brinker, telling him to collect all of the facts himself if he is so interested, and rushes out, then falling down the large staircase.

So, Leper's description confirms what Gene had tried to tell Finny in the past, but now, Finny finally believes it:  that Gene had knocked him off of the tree, with full intent.  This news is so angering that he storms off in a rage, and ends up falling down the stairs.