I have been mulling over two things, but will combine them here and hope that works as one message.
Things that could result from a major blast wave (either a plasma ball or a geologist's explosion or something else):
Yuri D's pronounced "vein outlines" on his arms.
Yuri D's gray foam from mouth
Yuri K's badly burnt leg
Other burns (Zina, for example)
Severe injuries of the ravine 4
Radiation
Teddy's tree having been blasted? and fallen in 1959?
If a major blast wave occurred, the ravine 4 would have been hit hard and all four of their bodies thrown by the impact, ending up where they died.
Lyuda: "The said damage was probably caused by an impact of great force causing severe closed lethal trauma to the chest of Dubinina. The trauma was caused during life and is the result of high force impact with subsequent fall, throw or bruise to the chest of Dubinina."
Zolotaryev: " ... The above mentioned multiple fractures of Zolotaryov’s ribs with hemorrhaging into the pleural cavity were caused in vivo as an effect of a high-power impact to the chest of Zolotaryov at the moment of his fall, squeezing or throwing."
Tibo: "...his death was the result of a closed comminuted pressure fracture ... The above-mentioned extensive comminuted fracture of the base and the vault of the cranium are .... the result of a great force with the subsequent falling, hurling and concussion of Thibeaux-Brignolle."
This does not explain, of course, why they had left the tent, but just for the sake of discussion, we could surmise they were all down in the area of the cedar / den / ravine, and a pressure wave exploded, killing at least six of them.
(Second thing I've been thinking about:) When we look at the photos and see Yuri D and Yuri K lying near the cedar, it is actually tricky to not automatically associate them with having died right there. Many people looking at this mystery have agreed that these two were moved after death, as their arms are lifted and frozen into the having-been-dragged position, and the liver mortis is upside-down. I have to remind myself frequently that they probably died somewhere else, maybe just 10 feet away, maybe 20 or farther, and then were dragged into their resting place.
So the ravine 4, Yuri K, and Yuri D were all hit squarely by a pressure wave, while possibly Rustem, Igor, and Zina were not in the direct line of blast. It was these three who tried to take care of the wounded and dead.
Not a theory, just musing.