I am super curious about this: big dive, probably not a stall (?) but either aircraft was airworth to carry on to destination OR at least the crew thought it wa

I am super curious about this: big dive, probably not a stall (?) but either aircraft was airworth to carry on to destination OR at least the crew thought it was. So all the controls and systems should be working properly, engines working. I am guessing they saw something worth avoiding? Birds? UFO (literally ... not aliens)? Another plane?