I would agree that it is too early to pinpoint what caused the problem, but I do think that some things might be eliminated. For starters, the generation of Airbus which was involved has an "auto" feature on its transponder which would have sent out an emergency signal, on its own, in the event of an engine failure. I get this information from Airbus pilots as I have no experience on that brand of equipment. All of my experience has been on Boeing equipment. having said that, I would think that the "glider" option can be disregarded.
When looking at the flight profile, it does resemble one of an aircraft that had experienced a depressurization. The procedure is to descend to 10,000 ft or the lowest safe alltitude (whichever is higher). In that area, the lowest safe altitude is something above 10,000 ft.
Another possibility could be a fire on the aircraft. Although this procedure would eventually lead to a descent similar to ths aircraft's, It would almost certainly be accompanied by a course change to "the nearest suitable airport".
While I can't speak to the quality of pilot training at this airline, some of the European low cost carriers have very inexperienced pilots.