Nobody knows yet, at least no reports of, why the firearm was discharged. Maybe it was accidental. Maybe it was deliberate. Mob justice is not the answer.
By the way, you link to Hong Kong Free Press - give me a break. I wouldn't call them Fake News but they are very biased.
They are similar to the NY Times who literally are fake news. The NY Times had the audacity to claim an 18 year old who was shot a few days ago was unarmed.
https://twitter.com/maodunzedong/status/1179797585638445057
Well everyone here on HackerNews can use their own eyes and see what the NY Times failed to see. An armed protestor (with the blue board) beating an officer on the ground and getting shot after he struck another officer who had his weapon drawn.
https://twitter.com/qingqingparis/status/1179736413534744576