Looked like a stray round struck some kind of ordinance and caused it to pop off. This can happen with bullets, all kinds of grenades, rockets, etc.. Main shooter using the scope and what sounds like an SVD (can't be sure about that) appeared to have other men firing with him, probably without a NV scope- just shooting into the area that the sniper is shooting. I think one of those rounds is what struck the ordnance that suddenly exploded there. You see all those black chunks flying around? Anything you see in this kind of vision that is black is much cooler than the surrounding environment. So when the explosion happens, a bunch of chunks fly up from under the ground, not having been touched by the sun all day- that's why they show up as black. Everything on the surface retains some of the heat from daylight. Notice the victim's rifles are all black- if they were fired a bunch of times, they would gradually become brighter in that scope.