Things that can cause it are an obstruction of the biliary tree, which can be due to stones, tumors (usually of either the papilla of Vater or the pancreas), benign strictures (due to chronic
pancreatitis or primary sclerosing cholangitis), benign stenosis of the papilla (ie, papillary stenosis), or a papillary diverticulum. Basically, when bile can't pass through, the duct becomes distended. So, an obstruction is likely causing it.