Most common cause of cholangitis -
Gall stones are the most common cause of obstruction in cholangitis.

Other causes are

benign & malignant strictures
parasites
instrumentation of the ducts and indwelling stents
partially obstructed biliary enteric anastomosis

Most common organisms cultured from bile in patients with cholongitis include

Escherichia coli
Klebsiella pneumoniae
Streptococcus faecalis
Enterobacter
Bacteroides fragilis
Viral infection
CBD stone
Surgery
Amoebic infection