It could be the c.g. I tellya!!! If that plane had a c.g. further back than it normally used, the thing could easily settle into a fairly stable deep stall if its pitch attitude was steep and then it ran outta air molecules for the tail to work with - just like the Viper.
Indeed, this was discussed as a possibility that the (somewhat) aft (cruise) c.g may have played a part in any instability, in early posts to the original R&N AF447 thread
The c.g. was
not further back than it normally used though, it would have been a normal aft c.g. for trim drag reduction, achieved using the tail trim tank.