1. Could be related to your field of study. Or they might suspect that you'll try to look for a job and not leave when you travel to the US. Remember that even if you were initially granted interview waiver, as a B visa applicant, you are still presumed to have immigrant intent, as per US immigration law.
2. Canadian PRs have a special privilege -- the visa reciprocity schedule for Canada applies to them, not the reciprocity schedule for their country of nationality. For example, Vietnamese nationals are only allowed 1-year maximum validity for B visas. But if they are Canadian PR, they could be granted 10-year validity B visa, despite having Vietnamese passport. Reference -- https://fam.state.gov/fam/09FAM/09FAM040308.html