Thanks buddy..
I haven't followed whole this stuff thoroughly since I wanted to be positive and focused on currently what I am doing than what I will be doing.
But below could be the reasons:
1) Background check by your local VO - If some documents are fake, they might reject you app. OR they have any doubts with docs you provided, they might ask you for additional documents(If everything genuine, no need to worry at all..)
2) Medicals - One of the applicant this year was suspect of suffering through TB, so his application was rejected.
But as I said chances are rare. So be positive and stay relaxed. Everything happens for a reason.