This does happen from time to time in places like this, & other than anybody getting hurt, it's a good sign for the river. It means that the water is good & clean enough to support the mayflies. Since mayflies are very susceptible to pollution, they are one indicator of water quality. It's just a shame that there had to be any accidents. Some folks think this was caused by the new street lights over the bridge.
There are about 300 different types of mayflies in Pennsylvania. That's a good thing for the fly angler & one reason why you need local info about the hatches when traveling across the state to fish in different parts. The bugs that hatch on one stream might not be the same ones hatching just down the road.
So when you see a massive hatch like this one, you might think that the fishing will be good. Well, in fact the opposite is true. If you find yourself trying to fish a humongous hatch, where there are so many bugs it looks like a white out, you don't stand much of a chance. That's because with all those naturals on the water, your odds of a trout taking your dry fly are much less. It's simply a numbers game. It might be neat to say you were there when it happened, but it's not always a pleasant experience, either. Make sure to cover your mouth, nose, & eyes.
I still think Hollywood is missing a chance at a great summer block-buster here. Think about it: a comment lands on earth & suddenly fly anglers are disappearing, then the mayflies start to swarm into town. Well okay, they can take it from there.
Take a look at this video showing some folks driving through the mayfly storm over the Susquehanna the other night: