Hello, because that carries a retain or seal that usually breaks especially if it is traveling through places where weeds abound, especially if they are fibrous stems.
It is easy to repair if you have minimal knowledge (amateur level for that) briefly: release the dish where the crosshead is going, they are usually 4 approx. Then there is a nut behind that dish, we release it and behind is the retain.
They usually go out with a screwdriver by making lever and you have to have a precaution of not making the lever where the seal is carried out to buy another equal, assemble the oil and fill with oil or replace it, good luck!