You know how piranhas, unlike the popular depictions of them viciously swarming after anything in the water, are highly mythologised when it comes to how dangerous they are, and the frenzied swarming is typically more of a scavenger behaviour than a predatory one? If something large like a horse or a cow or a human for that matter enters the water, they mostly give it space, especially if it shows signs of activity. Then, one of them might get brave enough to swim up and give it a bite. If it reacts, they flee. If it doesn't, then they swarm and you get the classic scene of them skeletonising an entire cow carcass. In the vast majority of attacks, you get a couple of bites and the person being bitten kicks up a fuss, the fish back off, and the person leaves the water.
Anyway, sometimes PMFs don't move when I give them a bite.