Taking its definition literally, a pilot is under somatogravic illusion anytime he is flying on instruments but believing his bodily sensors rather than his instruments. Since that is contrary to basic instrument flying training, there must be some extraordinary combination of circumstances for this to happen. Is a deceleration of 3 kts/sec sufficient explanation by itself, for two qualified pilots at the same time?
If an acceleration can produce a sensation of 9 degs nose up, why can't a similar deceleration produce a sensation of 9 degs nose down?
It can, but when it is caused by the attitude being actually 9 degrees nose up, the sensation would be that of level attitude.