My question is what causes positive opiate drug test?

Mainly opiates: There are false positives, but opioid tests are generally set at a very high level (like 3000 ng/ml) which eliminates most of the things that can cause false positives. The most common is poppy seeds, which really aren't giving a false positive, because they contain tiny amounts of a substance (papaverine) which is metabolized into morphine, so it is actually a true positive.

Only: if you are taking an opiate pain medication.