Diffuse peritonitis in acute appendicitis is caused due to:

Perforation of appendix in acute appendicitis commonly leads to an abscess cavity walled off by the small bowel loops and the omentum, forming a phlegmon. Rarely the appendix may perforate freely into the peritoneal cavity and cause generalized peritonitis. This usually occurs in cases of early rupture of the appendix as inflammatory process did not get time to be localized by the omentum and bowel loops. Usually the rupture of appendix is a late sequelae of appendicitis, usually occurring 48 to 72 hours from the onset of symptoms.