Otters are carnivores. They are primarily fish eaters, as a result of which they have VERY sharp teeth and strong jaws. A group of them, especially with any young, moving through an area would have a sense of space around them and regard any intrusion into that space as a threat. If the guy either inadvertently walked into that space or found himself inside the space because they were moving past him, he'd be perceived as a potential threat to be driven out of the space. Because we are so much bigger than the otters, they would attack in a bunch for their own safety. All of this is predictable.
While American otters do not form large social groups, they do sometimes form small bachelor groups and, of course, a female can have a litter which will stay with her until close to full adulthood. Doing anything that either kind of group would perceive as a threat in a situation from which they could not easily escape could result in an attack.