There is the argument that some randomness to the way insects (or other life forms) are influenced by all the factors insures the survival of the species.
True, but I don't think the "mass events" fit this description. Rather, the oddballs, and differences between streams, account for this. We all know you get that occasional confused sulfur that comes off in mid-April. Probably something wrong with him, but today's ailment can quickly turn into tomorrow's evolutionary advantage (see paragraph below). Also, there is variation in timing between streams, or different parts of streams. For instance, on most streams, a given hatch will start low and move upstream. If something happens to cause breeding to fail in one stream section, nearby areas where it was successful a day or two removed from the event succeed in reseeding the stream.
Regarding the ailment turned advantage, it's kind of like in humans, we always seem to have 1% of our population that is resistant/immune to a disease. Usually that "immunity" is the result of another condition of some sort, but what we call an ailment today can quickly become an advantage. Example: Sickle Cell Anemia. It's a recessive gene, and those unlucky ones with sickle cell anemia have a severe disease. If there were no advantages, the gene would be bred out of the population over time. However, the gene renders one highly resistant to malaria, whether or not the person is full sickle cell anemic or just a carrier. Probably still a disadvantage to be sickle cell anemic, but its a HUGE advantage to be a carrier in some parts of the world, which is why the gene is so widespread in those places.
And while we all agree length of day plays a huge part, that doesn't fully explain the difference between streams or stream sections. For instance, the difference in timing of the sulfur hatch from lower Penns, to upper Penns, can be close to a week, and another week for it to reach BFC. The difference in length of day between these places must be negligible. Likewise, here in Chester County, a hatch may be a full 2-3 weeks ahead of say, the Laurel Highlands, which are roughly the same latitude, hence same day lengths. There are other factors at play besides length of day.
But as fishermen, we're generally only concerned with the normal 99% of bugs in a single location. They all hatch together, they're not down there planning it out and choosing when to transform, so there has to be environmental factors which trigger the mass events.
