Because the picture shows a large bright explosion. You can see the ground that reflects the light and the fire ball in the air.
I am not an experts for photos and cameras and so on. But I can't imagine that such a photo can be created when a film is damaged by water or accidentally opened or anything in this kind. If I am wrong, please tell me.
I think the following happened:
They are all in the tent and preparing for eating. Someone (one of the ravine 4) is leaving the tent. Maybe to relieve him/herself, maybe to check something. He/she is seeing something not normal in the sky and shouting "hey guys there is something strange in the sky". The ravine 4 are interested, get dressed and leave the tent. The others are not so interested, they say "go out, look at it but close the tent". Semyon grabs his camera.
The ravine 4 are outside, watch something interesting in the sky. Semyon wants to make a photo and immediately when he makes the photo, the explosion happens. After this everything is chaotic, they don't manage to leave the tent through the entrance so they cut the tent and leave the area. They go to the cedar, make the fire, but recognize that it is not enough. The both Yuris die first. Igor/Zinaida/Rustem decide it is better to go back to the tent than dying like the Yuris (I think they are just weakened). So they try it but die on their way.
The other 4 think it is better to create a den. They try it but unfortunately something bad happens again (maybe another explosion with a shock wave, maybe the simply fell into a ravine, something like this). They all are weakened, fall down and in the end...die.
And when I remember correct, in 2017 a metal fragment was found in the area. Of course it could be something else, but it is another hint.
Besides this, I guess whoever was responsible for the explosion did not know that the hikers were there. Further I think only when the official search found the tent and the bodies, someone was informed found out "Were we accidentally responsible for this? Could be, so the case has to be closed quick and fast".
Other theories also assume that it was a military experiment but that the tourists were killed by humans. I can't imagine. Because there are no traces and I can't imagine that someone kills them, then goes into the tent, closes the tent, leaves everything as it is and cuts the tent from the inside.
I think it all was accidentally caused by a bomb test.