Why is the central tendon of diaphragm called a tendon?
A tendon usually connects a muscle to a bone, but not always.

According to InnerBody, the diaphragm muscle originates at the lumbar vertebra, lower ribs and sternum and inserts to the central tendon.


  The central tendon — a flat aponeurosis made of dense collagen fibers
  — acts as the tough insertion point of the muscles.


So, the central tendon acts as the diaphragm's own insertion. 

A similar example is aponeurosis of the external oblique muscle in the lower part of the abdominal wall (Brazosport.edu, Picture 10.11.a.).

A tendon is the connective tissue that is always a part of a muscle and it fixes the muscle during its contraction.

A ligament is the connective tissue that merely connects two bones or two internal organs and is not actively involved in their movements.