What would cause of high HC levels in an emission test?

Any number of problems can contribute to excessive unburned fuel in the exhaust. (elevated Hydrocarbons just means that the fuel didn't burn completely)

Check the air filter first, that's a common problem. Also, how long since a tune-up? Once the spark performance starts to diminish you'll see elevated HC.

Also, if your vehicle has an oxygen sensor it could be worn out, but usually that would be detected by a diagnostic scan.

These are just some thoughts on the subject. .