What triggers acute appendicitis when someone already has chronic appendicitis?

Obstruction. If you have the diagnosis of chronic appendicitis you should have an appendectomy. The appendix has no function and can only lead to an acute issue. Acute appendicitis is caused by an obstruction usually secondary to a fecalith or an inflamed lymph node. Occasionally it can be infected secondary to a tumor.
Not known. It is thought that inflammation in the wall of the appendix at its base near the cecum can cause obstruction to the appendix and overgrowth of bacteria and inflammation.