Generally, they've likely to have been 'charged up' before hand. Kept in the dark, little contact, then theyre suddenly released into the bright light, hundreds of screaming people, only one way to go. They're reacting instinctively and probably terrified.
The majority of injuries incurred to participants are either due to falling under the bulls, running into stuff and getting gored by an animal thats just reacting on instinct and defending itself from something it perceives to be a threat.
In bull fights, the Bull is subjected to much the same treatment, but is also wounded with spears and the like to both enrage and weaken it. Occasionally, the bull does actually win. But the entire spectacle is slanted heavily in favor of the man in the very, very tight pants.