I favor the terr theories, but let me reluctantly point out something else.
These were airplanes built in a country whose government did everything it could to punish or kill their own citizens who demonstrated any originality or individuality. They did this for over 70 years. They also systematically attacked innovation during this time period.
In the time since the Sovs fell, the remnants of the bankrupt system they left behind has been ruthlessly plundered by the corrupt.
If you combine the effect of airplanes built by people with no profit motive who cannot be fired (except for political reasons), with the fact that the Russians never had the funds or the cultural incentive to preserve the life of the individual by maintaining the planes the way that we do, a creepy picture develops.
You would have had to drag me kicking and screaming onto any Tupolev twenty years ago when there was actually better upkeep of these planes -- with Russia's fiscal mess today and the mafia holding most of what has been privatized, I wouldn't fly on one today 'for all the farms in Cuba.'
The planes are not maintained anything like what we would tend to project, from our own knowledge of the industry here at home.
Having said all that, I still think it was the terrs.