This year was a good year for horseshoe crabs. The weather was cooperative and the restrictions on horseshoe crab harvesting on the east coast the past few years has helped the repopulation of the species.
This is why you are seeing more horseshoe crabs stranded on the beach than in previous years. Most do not wash up on the beach, but come ashore to spawn and the waves turn them over stranding them on the beach making them vulnerable to predators. (shore birds, raccoons etc.)