I maintain the fact that there has been a problem with the thrust reversers on PW powered Boeing 767's but having just read that the aircraft plunged from FL330 to 19000ft then climbed up to FL240 before falling down to 10000ft, yes like a roller coaster, I would put aside the theory of thrust reversers.
Well, we have no clue. We still don't know what happened, the black boxes have not been analysed yet. We cannot say that the thrust reverser was the only cause of the crash. It is possible that BOTH of them had come out during the last minute of the flight, but I know that not just one of them would have been the cause of the disaster although the pilots were complaining of a thrust reverser problem on one engine of their 767. That particular aircraft did have its thrust reversers modified after the Lauda disaster. If that was really the problem, the FAA would have grounded all PW powered 767's until each one of them would be recertified to fly in airworthy conditions.
Because if it did happen on only one engine, like on Lauda Air, the aircraft would have spinned in either direction depending on which engine would have had the thrust reverser problem. And it did not happen with Egypt Air, the aicraft continue flying straight on course while suddenly loosing a lot of altitude. I would eliminate the idea of bomb threat. I would also elimitate the power loss of both engines because if the 767 looses all of its power at 33000ft, it can glide more than 100 nautical miles. I wouldn't put everything on hydraulic failures because the aircraft still had some of the control surfaces working properly, although it is possible that hydraulic lines controlling the elevator on the tail were severely damaged. Well the 767 never had hydraulics problems like the DC-10 had, but...it can always happen on just one flight no matter what aircraft it is. The other thing that is possible is that the autopilot system went down, thus loosing the ability of maintaining the altitude constant.
The Boeing 767 is an excellent aicraft with a good safety record in its almost twenty years of airline operation. It is one of the most efficient airliners in the industry, ETOPS (a certification applicable to twinjets for overwater flights) started with it and it is the aircaft flown the most on trasatlantic fligths. Boeing will continue building it for many more years to come.