Hurricanes form over tropical waters (between 8 and 20 degrees
latitude) in areas of high humidity, light winds, and warm sea surface
temperatures [typically 26.5 degrees Celsius (80 Fahrenheit) or
greater]. These conditions usually prevail in the summer and early
fall months of the tropical North Atlantic and North Pacific Oceans,
and for this reason, hurricane “season” in the northern hemisphere
runs from June through November.
CAPTION: Global map of hurricane formation timing based on warm sea surface temperatures.
CAPTION: Mosaic of all tropical storms that formed in the world between 1985 and 2005. As can be seen the majority occur in the Pacific Basin.
As for the stronger storms, the logical answer is that the frequency of storms during a given period is directly related to the likelihood that the storm will have more energy.
CAPTION: Chart illustrating the worldwide frequency of tropical storms (blue area) and storms of hurricane/typhoon intensity (pink area) by month of occurrence.
CAPTION: Frequency graph of Atlantic hurricanes and tropical storms by month with peak in September and 97% between of all activity taking place between June 1 to November 30.
Is the hurricane season getting longer per year?
Yes, it is getting longer according to research based on the first and last hurricanes of the year - (which I found as a result of this answer) - though my analysis that this research just reflects that we see more hurricanes now as a result of satellite imagery, not as a result of there being more storms.
CAPTION: Time series of the first and last tropical storm formation events each year in the Atlantic; red-lines (non-aircraft, non-satellite records); blue-lines (aircraft-based records included); green (saterllite based).