As a threshold matter, courts often apply strict liability for wild animals. A bull can be domesticated but retain a dangerous propensity justifying strict liability. This is particularly the case when a flank strap is tied around it. However, strict liability (even if it were to apply in Mexico) has a defense for assumption of the risk.
The negligence side of the equation is particularly strong. The woman’s legs appear tied to the bottom of the bull. While she may have just got them caught, they look as if they were intentionally tied — a uniquely bad idea. Then there is the long period in which they seem unable to stop the bull despite her obvious peril. They only appear to have ropes to handle such an emergency. When they finally stop the animal, they pull him on top of the struggling woman.
Of course the greatest display of negligence is allowing a clear novice to get on to a giant bull like it was some mechanical bull at a bar (which has an adjustable difficulty switch and an emergency shutoff switch).