The puff of smoke out of the back end at about 15 feet leads me to conject that it may have ingested (there is a lot of sucking going on) a bird which has cause a partial loss of lift fan thrust, but insufficient loss of thrust to trigger the auto eject which is triggered on a sensing of high rate negative pitch. The pitch rate we see is not as high as I have been led to believe we would see if for example the lift fan drive shaft disintegrated. Maybe a bird did some damage.
In regards to the arm restraint question earlier, the Mk16US seat has arm, leg and head restraint (to protect from neck injuries while wearing the HMD)