Like yourself, Gary, I was strongly of the opinion that a common failure mode took out all 3 landers while they were still coupled.

However, at some point last year I read an analysis of DS-2 failure that pointed out that the permafrost of Mars was a major problem. They were designed to land in sand or gravel - non-cemented material which casn be displaced and absorb the kinetic energy of impact. Permafrost makes absolutely excellent concrete to the point of breaking picks in Siberia when you try to dig into it.

It was only later, post DS-2, that Odyssey proved such extensive and shallow permafrost, so they were not designed to land in such materials.