A real quick scan of incidents shows several of these clusters. For example, 1 base fatality and 2 in a single canopy collision in Europe over the weekend of 6-7 May; in 2005 over the weekend of July 4th, 1 TI killed in a collision, 2 swoopers in a single canopy collision, and a base fatality in France. Memorial day weekend might have stood out more because of the unusual nature of one incident and the fact that a couple of the people who were lost were well known and their passing generated a lot of comments. As Billvon pointed out, there's no common element here. The people involved were current, they were experienced - something went wrong. I'm trying to think of something eloquent to make sense of it, but all I can think of is BSBD.