Probably 99 times out of a hundred if a turbine is going to scatter it’s going to do it under the highest loads at rollout/liftoff and during ascent. Toodling along at 30+K feet is like being on cruise control for them. I’d posit the protective fragment shroud is titanium or some type of ceramic composite that’s some value more brittle at cruising altitude than at lower elevation where the majority of failures are expected to occur. A failure at cruising altitude, under this hypothesis, would incur more damage, but probably still within design parameters where the aircraft stays aloft and controllable.
Under normal situations...incident is at highest load..instead..it happened during cruise control. The air lines as of last year were instructed to make Ultrasonic Inspections of fan blades of engines. Move was prompted by a report of a fan blade failing and hurling debris...On Southwest Air flight **determined explosion due to Metal fatigue.