What Could Be The Reason ?

well , I'm personally think that they had a fight.
And I think is Wong Mew Choo who broke up with Lee Chong Wei not Lee Chong Wei.

As I know , I watched the 'Chat with VVIP Lee Chong Wei' in August last year , it seems that Lee Chong Wei is very loud and rude where Wong Mew Choo is soft and sweet.

But Chong Wei is still indeed a very nice guy too but sometime he maybe too loud.

As you see every other news , chinese , BM or even English . Wong Mew Choo was alone and was not being cared where all the psychologist,physiotherapists, masseurs, technical analysts, sponsors and coaches were surrounded with Lee Chong Wei where Mew Choo is just at the other corner sitting quietly. Here are the proofs ,

'The 2007 China Open champion cut a forlorn figure during her preparations for the match after her arrival in China on Sunday'
-adapted from The Star

Forlorn means lonely , not being cared about.

结束了一个多小时的训练，李宗伟和黄妙珠分坐地板两端进行着放松，全程没有任何交流。心理医生、物理治疗师、按摩师、技术分析师、赞助商和教练将李宗伟团团围住，黄妙珠则一人安静地拆着右膝的绑带。
-adapted from sports.sina.com

Translation in English for that is..

Ended more than an hour of training, Lee Chong Wei and Wong Mew Choo am sitting at both ends of the floor for a relaxed, full without any exchanges. Psychologists, physiotherapists, masseurs, technical analysts, sponsors and coaches will be surrounded by Lee Chong Wei, as for Wong Mew Choo is one who sits quietly at the other end and dismantled her right knee band.