Your visa to Canada can get rejected when your passport is invalid, it is essential to make sure that your passport will not expire during your trip. Another reason could be having a criminal record, of course non-serious offences such as a speeding ticket do not count.
When a tourist has some criminal record, it may be necessary to send in some additional documents. Visas to Canada often get rejected when tourists fail to send in the additional documents in time. Your visa can also get rejected when you apply too many times at once.