Happens from time to time when the fish is too large. Tradition started over 40 years ago by a frat that later lost its charter at UNH, for reasons that one can only imagine (think 'Animal House,' the film). But, as the story goes, two frat brothers were allowed back on campus only for home men's hockey games, and only for as long as it would take for UNH to score its first goal, after which the fish throwers were escourted out of the building. Not sure if still the case, but a local fish market would donate the most rotten fish that they could no longer sell, with some breaking apart as they hit the ice. Obviously, the youthful fish throwers now must be second or third generation, as the original frat bros would be in rocking chairs watching games on NESN now.
Greg probably can elaborate with corrections and more details.