If you owned it and were filming, why on earth would you be laughing instead of immediately intervening?
Has another video emerged showing which of the 2-3 witnesses was laughing, who they were and what they were laughing at?
If not, then that's dangerous speculation. If there is another video that means you are commenting on a proven fact as opposed to conjecture then I don't know, I can't understand that if that's what happened.
In terms of why nobody intervened in the moment - the human threat response is fight/flight/freeze. Lots of people freeze in a situation like that. You can hear that the rider is scared, why wouldn't other people have been?
Blaming the rider, blaming the filmer, blaming the other people in the background all sounds like victim blaming and lots of the narratives you see about rape victims are being used here. "Oh why didn't she fight back, why didn't she stop him, why didn't she scream". Well why was he a rapist and why was Dujardin beating a horse - those are the questions that are relevant.
You can't even see what was happening off camera yet people are happy to whip up a mob saying vile unproven things about them.
The person to blame is Dujardin. Her behaviour was wrong. She is shown in the video, has admitted it was her, and she deserves opprobrium.
Speculating about, and apportioning blame to, individuals who aren't even in the footage is really dangerous. Speculatively naming other people as being involved is also inappropriate and potentially dangerous.
If you degenerate into vigilante mob behaviour, you no longer have the moral high ground to censure Dujardin or press for real change in the industry.