I have seen people try to halt the closing door or even pry the doors open after they have closed (one sees this frequently on the TTC subway, and that practice may be transferring to GO). The result maybe someone clinging to the doors and standing on the step as the train starts to move.
The CSA may not have clear line of sight to both ends of the train. The doors will alarm, but unlike TTC where the train will reliably not start while the doors are abused, the connectivity between doors, control, CSA, and engineer may feel a bit less fail-safe. Crews will understandably take the safest course, ie not start the train. There is then a starting match between the CSA and the passengers on the platform. Again, crews may feel safer if they "blink" and reopen the doors instead of dissatisfying a customer who may file a complaint. This is where the inappropriately aggressive customer clearly wins.
If you have ever taken GO at night after events downtown, where platforms are packed and the urge to cram on and just push people inwards is great...... GO customers are very untrusting of GO - and maybe a bit pumped or intoxicated - so getting on board and not waiting for the next train (which may be a long way off) is just too compelling. I suspect the same happens at rush hour. GO Security people seldom intervene unless the doors are unable to close, and then are more likely to urge people to push in than to get off and wait.
The issue is clearly bad-behaving customers and not attributable to the equipment or the operators - but as I say, we seem to treat bad behavers with kid gloves.
More innocently, people coming off the stairs will impulsively run towards the train when they first see the doors about to close. Running on those narrow platforms is inherently unsafe. The one minute pause between door closing and train moving may have some benefit in calming movement on the platform.