for the same reason that I ran into the road in front of an oncoming car. I didn't think my speed or agility was a coat of armor, and I accomplished my goal without getting hit or causing worse injury to anybody. I simply could not leave the escaped toddler to run out in front of an oncoming car. I could see her mother's panic and hear her mother's silent scream in my own heart and head.
It's the same choice that some people (not me because I am deathly afraid of getting burned) will make on the fly, to run into a burning building to save a screaming kid.
Entering that situation doesn't mean that you will run in and start shooting. If you can run in and shout loudly enough, you can freeze the situation in place and prevent injury. You don't need to know who the aggressor is. Just scare *everybody* into dropping, keeping hands over head, and obeying your orders.
(I realize as a horse trainer, I sometimes see things a little differently than others. It's not that I go around routinely shouting at my horses, lol. It's just a skill that you develop to make yourself bigger and louder than anything a panicked horse is freaking out over, to keep them from making a bad situation into a disaster. It can happen with flighty youngsters in a heartbeat. My gut tells me if I can go from quiet and laid-back to bringing a half-wild, panic-stricken thousand pound horse to a dead halt by shouting, I can do the same in other emergencies. And I am not special. If I can do that, so can others.)