Usually when there is a "yes" answer to any of the statutory questions, or when they need more details or clarification regarding past travels or personal history. May be something as innocent as an applicant putting the wrong dates that they feel need to be corrected, or they may be on the lookout for concerns about an applicant's background in declared military or government activities or prolonged stays or numerous repeated visits in certain countries not explained by employment.
If you answered "no" to all statutory questions, and there are no potential security risks in your travel history or personal history, you likely won't be requested for Schedule A.
Schedules 4 and 4A are only requested to provincial nominees because they somehow do not include this at the start of the eAPR even for those who got an ITA under PNP.