Pancreatitis creates a severe inflammatory state in the organ that extends to its surrounding. When the inflammatory mediators reach the internal linings of the abdominal cavity (the peritoneum), the muscles of the abdominal wall become tense around the inflamed area. This is also known as muscular guarding, and it is detected in the physical exam as a difference in muscle tension around the painful area.
It might be a bit difficult to detect muscular guarding and differentiate this symptom from the ordinary muscular defense reflex people would have when touching a tender area. This symptom is common in almost any inflammatory condition of the abdomen, including pancreatitis, diverticulitis, ectopic pregnancy, and many others.