What causes the bilirubin to build up?

Dead blood: It usually depends on age genetics and liver disease. In newborns the liver is not well developed so it cannot process the bilirubin which develops due to dead blood needing to be recycled. In adults there is a genetic cause which is of no significant concern - gilbert's syndrome . But, the other usual culprit could be liver disease due to alcohol abuse, hepatitis, or chronic meds use.

Three mechanisms: High bilirubin that is almost all unconjugated means hemolysis (tearing up red cells too fast) or gilbert's non-disease. If it is mostly conjugated, the bile ducts are obstructed, perhaps by a stone or tumor. If both are way up, probably the liver cells are being damaged.