What causes leaves to be hydrophobic?
This is due to presence of a hydrophobic chemical called as cutin present at the the aerial surfaces of plants.


  Cutin is one of two waxy polymers that are the main components of the plant cuticle, which covers all aerial surfaces of plants. The other major cuticle polymer is cutan, which is much more readily preserved in the fossil record,.[1] Cutin consists of omega hydroxy acids and their derivatives, which are interlinked via ester bonds, forming a polyester polymer of indeterminate size.




Source: https://en.wikipedia.org/wiki/Cutin