The leaves and even the stalks contain Oxalic acid. This is lower levels in the stalk than the leaf and nowhere near enough to be dangerous. The Oxalic acid in itself is not a problem, but the leaves also contain Anthraquinone Glycosides. It is thought that the combination of the two may be the reason for the leaves to be poisonous.
Why is this Anthraquinone Glycoside found in the leaf and not the stem? I can only assume that it is not produced in the root or stem, but only the leaf. There are many plants which produce toxins in the leaf for protection against insects and insect larvae.