DF,
If you consider the number of Chieftains in the "working" GA fleet, the hours they do each year, the rough treatment received doing freight runs (not a dig at the drivers, just the "mission profile") and the average age of the Chieftain fleet (roughly 25y.o.) then it is not surprising that there is an elevated number of Chieftains in the stats.
Add to that the nature of GA and the less experienced pilots finding their way (again, not a dig - I was one and might be one again) and what would you expect?
I wonder:
*If other aircraft types were substituted in the Chieftains role (say a C310) would they not feature similarly? and
*Compared to hours flown per year, does the Chieftain fleet as a whole have more incidents/accidents than any other type similarly employed?
Have a good weekend!