Why did Hurricane Erin strengthen so quickly, and how unusual was this compared to other storms?

Hurricane Erin strengthened quickly because there was nothing to impede development, such as strong wind shear or enhanced Saharan dust. The ocean waters are warmer than normal, and this provided ample fuel for rapid intensification. It is becoming evident that rapid intensification is a common occurrence in Atlantic hurricane seasons, so, historically, this is unusual. However, in recent seasons, we've seen more storms rapidly intensify, so it is becoming less unusual with time.