What could be the causes of an elevated liver function tests?

Many things... Elevated Liver function tests may occur for a variety of issues from Fatty infiltration (hepatic steatosis), viral illnesses such as the hepatitis viruses, Alcoholism, increased use of medications that metabolize through the liver (such as Acetaminophen), cirrhosis, biliary disease, tumors, gallbladder disease, etc, etc. Get an evaluation for the most likely cause of your LFT elevations.
A sick liver. Lft's (liver function tests) may be elevated due to hepatocellular disease, biliary disease, infiltrative disease (fatty liver)... Some lft's are nonspecific, and as such may elevate due to problems with organs other than liver (bone, gut, muscle, heart). Chronically elevated lft's, lft's trending upward, or acute profound elevations in lft's are most worrisome. Don't ignore any lft elevations.