SAA leaves their passengers behind

Had a flight out of Gaborone back to OR Thambo, the flight was due to leave at 5:20 pm gates close 20 minutes prior To Departure As Usual. I get stuck on a call, however get to the boarding gate 25 minutes prior to departure and they have off loaded my bags and the plane is leaving. They said they announced my name however the Gaborone airport speaker system has a problem and you cannot hear anything that is being announced. How can a flight leave 25 minutes prior to departure time when the gates close for boarding 20 minutes prior and they have already unloaded my bags. I am a single mom who has to get back to her children, I have had to book an air botswana flight home as it is the earliest next flight, incurred international telephone costs to try and make arrangements for my children and an additional flight cost. When checking into Air Botswana I asked the attendant is they also leave earlier than what is set on the boarding pass, she said \we don't leave our guests behind"..... just saying they must have this happen frequently. """