Frenchie. The plane must have encountered "still air turbulence". I have experienced it twice and it is really very scary
. The aircraft just drops 1,000's of feet and then stops with a bump when it reaches the bottom of the air pocket, or whatever. It doesn't dive but just drops level. Passengers and cabin crew have been known to hit their heads on the ceiling and I think, at worst, there have been broken limbs. Incidentally both my experiences were with BA.
These days it is a very rare event.
Ken
My god. I just reached the later detailed post about this. After all that "still air turbulence" would be a pleasure. I can sympathise with your alarm and I am glad that you all came through OK.