Of the 30.89%, I suppose the vast majority would be those who have not complied with CIC's requirements. This could be by way of job description not meeting the NOC requirements (i.e. post 26 Feb 2008 applications), leaving fields blank, not signing documents, insufficient points, etc.
From the reading of other threads, I also see that applications are also rejected or returned for reasons outside the typical. Among those I have seen are;
a. Problems with Credit Card payment deduction; and
b. Perceived lack of language proficiency.
There may be others and, while it may represent a smaller percentage, it would be good to identify them, so that future applicants can avoid these.