The number of Atlantic hurricanes can vary tremendously from year to year, due to factors such as rainfall in western Africa or the state of El Nino. Another factor is the average temperature of Atlantic waters.

Over the last 100 years or so, the Atlantic has gone through periods of relative warmth and relative coolness. Broadly speaking, the cooler years were from around 1903 to 1925 and then again from 1971 to 1995. The period from 1926 to 1970 and the years since 1996 featured relatively warm waters. The map on the left shows the tracks of the major hurricanes that threatened the East Coast during the cooler water periods, while the map on the right applies to the warmer water years.

Clearly, there's a big difference. You can see that more major hurricanes threaten the Eastern U.S. when the Atlantic is in its warmer phase. It's been in that warmer phase since 1996, and that's likely part of the reason that Atlantic tropical activity has been above average six out of the last seven years.