There's a movie that tells the story.
Robbie Benson starred in it.
The singer said she never knew why he jumped, so the movie is just one interpretation.
I agree. Watch the movie, and you will get your answer.
That's one idea. However, Wikipedia, Ode To Billy Joe
(note the change in spelling) says :
When Gentry and Raucher got together to work on the screenplay, she explained that while the song was based on an actual event, she had no idea why the real person who inspired the character of Billie Joe had killed himself. Raucher thus had a free hand to pick one.
So while the movies explanation is a valid opinion, it is not necessarily the only one (as pointed out), and Gentry herself, said she had no idea why Billie Joe jumped.
Sop please give your idea as to why he jumped and what was thrown off the bridge and why. If you think the movie is correct, say so and why you think so. Don't just give the off hand remark the movie says why; back up your reasoning.