Parent visa Dropbox but 221g slip asked them to appear for counseling

Hello, My parents visa extension is due and they are eligible for drop box. They submitted application in drop box. But in couple of days they asked to collect the passport with 221g blue form and asked them to appear for counseling. They returned passport. Any idea why this happened. And what is the purpose of counseling. I greatly appreciate your answers. Thanks in advance

Something in their application has caused them to raise a red flag, which they would like to clear out in a face to face interview.