this incident involved a russian airliner on a flight between 2 russian destinations but overflying another country ie ukraine. unlikely the russians would be able to cover up anything untoward, that would require the connivance of the ukrainians.
the ukrainians did shoot down a russian tu154 in 2001, but it is unlikely that this has happened again.
the tu154 first flew in 1968, but the aircraft in this incident was new in 1991. this was the 63rd tu154 to be a total loss, but that must be set against the number built - 1000, and many are still in service. the plane doesn't have a bad record. pulkovo aviation joined IATA in 2000; they have had 2 previous, accidents this latest one being the 3rd. the airline dones't have a bad record, which probably wouldn't be the case if they were habitually poor maintainers.
on the balance of probability, the cause of the incident is likely to be as speculated, ie lightning strike or severe turbulence.