Besides recent consumption of alcohol (as mentioned), many prescription drugs and over-the-counter medications can have an effect on liver enzyme tests as well.
If you have HCV, fluctuations of AST and ALT can be expected.
Elevated liver enzymes is a warning flag and should be checked out. With elevated enzymes, it is normal to check for hep A, B, and C. These are simple blood tests and should narrow down the possible reasons for elevated liver enzymes either way.
Because something is causing the liver cells in your liver to die. Do you have hepc that is a likely culprit.