Many. Viral hepatitis, portal vein thrombosis, congestive heart failure, cirrhosis of the liver (although liver can be small in cirrhosis) are a few of the many causes. Cirrhosis of the liver can be a result of alcohol abuse, long-term viral hepatitis, many other causes.
Infections, cancer. Certain infections, early stages of some hepatitis, blockage of the common venous drainage and certain cancers like lymphoma.