I'd think that after a celebrity does, there would be an increase in the number of edits / length of an article. Wouldn't this skew the numbers, increasing the "celebrity" statistic for everyone that has died that year (and hence making the year seem more extraordinary than it is)?
It would be interesting to do the same analysis for 2015 and previous years, using wikipedia snapshots at the end of each year, and see if those years also appear extreme.