That is exactly it. Cushing got caught with high HCG levels in his system. HCG is a hormone normally produced during pregnancy. In fact, most over the counter pregnancy tests are testing for HCG. It can also be used to induce ovulation for women having fertility issues. My wife and I became very familiar with this stuff when she got pregnant with our daughter. The point is, it high levels of the stuff would be odd to show up in a man's body.
Peter King had a good thought about Cushing in his MMQB column. If, Overtrained Athlete Syndrome was a potential cause for Cushing's positive test, wouldn't you think that other football players would have been found with high HCG levels as well? They certainly would be good candidates for it. But the fact is that "no player in the history of the NFL substance-abuse program before Cushing tested positive for the higher level of hCG." He's the only one.
Sam Bradford aces first test; Texans fight for Brian Cushing - Peter King - SI.com