The most important thing to consider regarding a change in the volume of motorcycle accidents, or any other type of highway motor vehicle accident, is traffic volume, which in this case would include both motorcycles and other types of motor vehicles.
Probably the next most important factor to consider is the type of highway, especially freeway vs. non-freeway. Then you have the condition of the roadway (wet, dry, icy), the condition of the rider (sober, drunk, asleep) to consider.
In this case, the hardest thing you have to deal with is the relatively small number of motorcycle accidents — 14 accidents a year is about one every 26 days, or one a month. Not easy to spot a trend with those kind of numbers.
Even so, there apparently is a trend with respect to the average age of motorcycle riders — they’re getting older and more inclined to riding mostly on weekends. I followed one up the Hill a couple of weeks ago and I fully expected him to go down on every curve.