Maybe whatever he tested for wasn't vegan and he doesn't want to get shunned from his social circle of vegans.
Is there any animal byproducts in any performance enhancer? Srs question.
Maybe he took too much Tylenol the night before. Maybe he took a sleep aide the night before. Maybe he took a drink of one of his teammates contaminated bottles and doesn't want to out that teammate? Or maybe he cheated.