Distiller here,
you've been had.
What you're seeing is precipitate dropping out of solution that is no longer soluble in the aqueous juice with alcohol added (read: no longer aqueous).
Why organic juice? Less filtering = more dissolved solids.
When you add the alcohol, you're rapidly decreasing the solubility of the juice for water soluble particles, thus the precipitate.
This is the exact reaction you get when you add water to high proof spirits like Absinthe, Gin, and even some whiskies, but the opposite manner (alcohol soluble to aqueous solution with added water).