The most common explanation for a huge drop is just "one hit wonder": A name got popular (e.g., because of a song or a TV show) in one year and forgotten in the next. The exception to this rule is the name
Hilary (for girls). Recommended reading is this essay by
Hilary Parker:
http://hilaryparker.com/2013/01/30/hilary-the-most-poisoned-baby-name-in-us-history/