That's an astropectinid, and like many other marine faunas have "blooms" that sometimes simply cannot be explained. The one in the photo looks healthy however, so that rules out some stuff. Stress usually diverts these things offshore, not on the beaches. The "big storm" theory is a good one in the winter months, and can be confirmed by looking at the amount of fresh wrack and other faunas.
Best bet is to wait for the university clowns to come down and look things over. This isn't something Parks & Wildlife or the equivalent in Florida is exactly an expert on. You need an echinoderm person, water quality specialist, and a local marine ecologist.