Fishing Rod takes durability when it hits a block.

This is working as intended. If you use a tool on a block its not meant to be used on, it takes double durability. In the case of Fishing rods, if the hook hits a solid block instead of water, they take durability as well

• Casting in water and reeling it up without a catch, reeling before the cast is complete, or missing a bite and leaving the line in the water does not reduce durability.

• A double-durability penalty applies if the hook lands in solid blocks, either under water or on dry blocks (if this occurs under water a fish may still be caught).

• A triple-durability penalty applies if hooking an entity.

http://www.minecraftwiki.net/wiki/Fishing_rod#Durability