The reason prolonged fasting raises bilirubin levels is due to downregulation of the polypeptide transporter of organic anions in the hepatocyte cell membrane.
The increase in levels of bilirubin can also be sometimes caused by an increase in the caloric intake during fasting.
Bilirubin is a yellow compound that is a breakdown product of heme from hemoglobin in red blood cells and fasting can also cause an increase in bilirubin levels because of reduced intestinal mobility.
If your level of bilirubin gets to a too high of a level then it could be dangerous so be very careful when fasting although fasting has health benefits including weight loss.
The fasting when done too much can be harmful to your health.