I don't know the statistics. You can't predict the storms. As a coastal person that keeps watch, later August through early October seems to be the most active off the south east coast.
Dave said this quite well about them changing routes and ports to avoid storms.
The main thing if bringing a group is to educate them about cruising at that time of year. Yes, it costs less money, but is in storm season and the ports are not guaranteed etc.
Hope this helps,