Generic list of reasons for rejection (based on what I have seen on this forum):
1) Incorrect/Expired PCC
2) Medicals which have not been shared with IRCC
3) Employer reference letter does not match job duties on NOC
4) Work Experience claimed during Co-Op timeframe
5) incorrect NOC selected
6) Language results do not meet threshold
7) Copy of expired Passport submitted
8) For CEC category, days worked when ITA less than 365, even though days worked after submitting PR application greater than 365
The list is not comprehensive and there are other reasons as well.
