After getting stuck in a strong storm this spring on Penns Creek a buddy and I suddenly saw a handful of nymphs on the trail. Went and looked it up and it turns out some people think a storm triggers them to crawl out of the water. The pertinent info from this page is below.
https://entnemdept.ufl.edu/creatures/misc/eastern_dobsonfly.htm
"When ready to pupate, hellgrammites leave the water and may pupate close to the water or travel up to 15 meters or more in search of a suitable site for pupation (Mangan 1994) - typically under a rock, log, or some type of debris that serves to maintain a moist environment. Exodus from the water of full-grown hellgrammites in a given location is fairly synchronous (within a few days). Voshell (2002) states that local residents along Virginia rivers report that thunderstorms trigger emergence of the hellgrammites - a phenomenon known locally as "hellgrammite crawling". It is believed that the behavior is stimulated by the vibrations from the thunder."