It may have been an early L.S.D test, by the Russkies.
On unwitting subjects.
Ergotism wasn't exactly unheard of at the time, and I think if it had been present, then the autopsies would have picked up on it. But whatever the exact cause, I'd bet one of my knackers on it being something to do with them all being Royally fucked up on some kind of Drug that none of them were accustomed to.
And it's not like the Russian Govt had any more scruples than the US Govt when it came to testing Drugs on unwitting citizens, just to see how fucked up it got them.