Another thought...
If they stepped on a landmine, why is there even a need to cover it up? It's not the authorities fault.
If they entered some military booby-trap, maybe the local military would want to cover it up but according to what Nigel raised above.. the second most senior person in the SU was involved, surely if a local military branch is to blame they could just put them on trial and save face.
Moscow's involvement would hint at some national security concern. Here is some wild guess.. what if the deaths are somehow actually related to nukes. Not a nuclear test (we know there hasn't been one at the time due to the treaty with the US, and it would be detected from abroad), but what if they just came across an abandoned nuclear facility or uranium mine? Maybe they developed radiation sickness days later, even if it's not the ultimate cause of their death, the authorities might want to cover up...
Why? Because they would be afraid the incident might spark widespread anti-nuke protests. In the previous year, 1958, the anti-nuke movement was already gaining momentum in Western Europe and maybe they were afraid it would spread and undermine the Soviet atomic programme and by proxy also their competitiveness with the US military?