The incident happened at around 6pm as the parade ( Which started sometime after 2pm ) was ending, which probably explains why hundreds are walking in the road as the police/stewards start removing barriers and opening them up again. In another video I've seen you can see barriers stacked up.
It looks like the car has somehow ended up amongst the crowd, probably by thinking, "Oh the roads open I'll cut through here" then realising he's fucked up as he's in the middle of hundreds of people. He starts beeping his horn to move them and people get angry thinking "What the fuck is he doing driving here?" He starts trying to nudge them out of the way increasing their anger and also his frustration and anger. As he bumps people they start attacking the car, one person opens his door. One man behind him is especially angry trying to kick his back window in. The driver reversed into him pushing him back. He then picks something up and throws it, probably smashing his rear window ( Visible in later pictures as the car is stopped. )
It escalated very badly from there.