???? i really dont kno lol, i have a guest but im going to see all of them below arent i, so yea....comment those wrong answers! lol jp
agreedMy Guess...
Zetsu's are made of Hashirama cells and i bet that the mass that holds Zetsu together is made of wood + something else? (havent done research)
now, however when they get in contact with Yang-Natured chakra the nature power is too strong for the Zetsu Clones, resulting in evolvement/getting out of control...
do you remember when naruto learned Sage Mode? you need the perfect amount of Nature Energy in order to hold the "Sage Mode" but what happens if you can't control it? if you charge too much Nature Energy? yes... you would turn into a frog and later turn into stone?
i think it's pretty much the same thing, Zetsu's can't control the overwhelm of Yang-Chakra (?)
I think it is because naruto have only yang chakra of kurama which is physical energy and wood release is likely to be yang nature.
I am not sure