We now have video of the Soyuz failure, and it was booster separation that caused the mishap:
On Thursday, Russian space officials held a news conference to lay out their findings into an October 11 accident that involved the launch of a Soyuz FG rocket and its spacecraft. The crew of NASA astronaut Nick Hague and Russian cosmonaut Aleksey Ovchinin escaped safely, but the rocket was destroyed.
The problem, the officials said, boiled down to a “bent” sensor on one of the rocket’s four boosters that failed to properly signal stage separation. This caused one of the booster stages to improperly separate from the rocket, which can be seen in the video released by the space agency. This booster then struck the core of the rocket, causing a significant jolt and triggering one of the Soyuz spacecraft’s automatic escape systems.
According to the officials, the sensor rod was bent by a little more than 6 degrees, and this happened during assembly of the rocket. The Russian space corporation, Roscosmos, has classified this as a handling error. To fix the process, Soyuz rockets already assembled for launch with their booster packs will be disassembled and reassembled to assure that similar mistakes have not occurred.
It should be noted that, by the standards of man-rated boosters, the Soyuz is quite safe, and the escape system functioned as it should have.
Still, a bummer for the astronauts/cosmonauts and ground crew, no doubt.