This is not an uncommon mistake for pilots in training to make. It sounds strange, but you have to keep in mind that this maneuver is done with no outside visib

This is not an uncommon mistake for pilots in training to make. It sounds strange, but you have to keep in mind that this maneuver is done with no outside visibility (that's the reason for the go around). So the pilot is only looking at his instruments. The first officer had to do several things at the same time, and for a few seconds did not monitor the aircraft pitch attitude well enough. In a jet the difference between a normal (about 600 to 700 ft/min) descend or this rapid descend is only something like 5 degrees. So it's easy to get this wrong if you look away from the instruments for just a few seconds. Especially if you're changing configuration (flaps) and power at the same time, because both have an effect on pitch.