I don't have any knowledge about Virgin Galactic's trajectory, but in general rocket trajectories are chosen such that no matter where the rocket fails along it

I don't have any knowledge about Virgin Galactic's trajectory, but in general rocket trajectories are chosen such that no matter where the rocket fails along its flight, it will either fall entirely within the published keep-out zone (and cause no harm to humans, since the zone is cleared before launch), or are going fast enough to break apart during reentry and will not cause harm to humans (since the majority of the vehicle is just particles in the air, or very small pieces.) Obviously Virgin Galactic's vehicle can't be terminated since it's crewed, so I'd expect that the keep-out zone is designed so that at any point thrust can be cut and the vehicle will remain within its allowed airspace. Again, I have no knowledge about the flight corridor they are using, but if we imagine it's a two-level volume with a large (but low-altitude) cylinder, and a small (but high-altitude cylinder), it's possible that the trajectory extends beyond the small, high-altitude cylinder while still above the ceiling of the large, low-altitude cylinder. If that's the case (and I'm definitely not saying it was!), then if the vehicle velocity has too much of a horizontal component during the engine burn, it will exceed the limits of its allotted airspace unless the engine is shut down (in which case it won't be going fast enough to exceed the limits.) Again, this is probably not exactly what happened; it's just an example of how an early engine shutdown could prevent them from violating airspace.