My very anecdotal theory is that those seeing humans frequently get used to us (up to a point) and usually just end up hissing when we get too close.

Where I live geese are omnipresent in parks and people are cycling, jogging and walking through their flocks. They will certainly act inquisitive, hiss and act like they own the place, but I've never seen them attack.

However, maybe a flock in a place less busy with humans and less used to seeing them would be more willing to attack.