The person telling you doesn't know why, and there is no way for them to directly check, but there are a variety of reasons reservations get canceled, reasons they airline doesn't want to pass on to you. Top of the list is being pumped for an airline employee who needs to be somewhere. Then come premium customers such as government agencies/police/military. Then rich people (premium travelers/club members etc). When such people get a flight at the last minute, they are bumping someone else. That someone isn't told why.