As posted on the official site:

"I was substance-tested randomly by the NFL during the 2009 season. The results of those tests indicated the presence of a non-steroidal banned substance. The League acted to suspend me based on those results, which I challenged in my appeal of the suspension. I believe we presented compelling evidence during the appeal process to challenge the test results, and I disagree with and am disappointed by the suspension. Bound by the decision of the League, I regret the situation it presents to the Texans' organization, my teammates, and our fans. My dedication to a championship season in 2010 continues undeterred."

So...some unknown non-steroidal substance (as Jay Glazer said) triggered the test, and Cushing's "compelling evidence" wasn't enough to persuade the League. We'll never know the whole story, but it sure would be interesting to know what the substance was and why Cushing's people thought they had enough to challenge the validity of the results.