An article on a US football (well, soccer) website sheds some light on the situation. It seems young Devann was sent packing for that most heinous of crimes - eating an egg sandwich.
Here's the extract from the article:
"The coach sent you home for eating an egg sandwich? “It was a misunderstanding,” Devann tells me after he returned. “I missed breakfast and grabbed something before training - a sausage and egg sandwich. I went into the training room and was eating, and the U19 coach and technical director came in, noticed what I was eating, and was like, ‘I’ve had it. You’re not training today. Go back to your hotel.’ I got a phone call from him later that day. He said I did well on the field but off the field I didn’t show enough enthusiasm, that I didn’t want to be there. I mean, I missed Christmas, missed New Years. I wanted to be there. But he let me go.”"
Of course, this is only his side of the story so it could be utter nonsense. No doubt it'll be brought up in the future if he turns out to be a superstar.
The full article can be read at http://www.thisisamericansoccer.com/tias-special-guests/the-curious-case-of-devann-yao/