It is presumed that the combination of full-power acceleration and the lack of visual cues from outside the aircraft caused a degree of spatial disorientation (the 'somatogravic' illusion whereby the brain mistakenly interprets acceleration as increasing pitch-up because of the similar effects which both motions have on the ear canals).
This false 'pitch-up' sensation probably prompted the captain to push the nose of the aircraft downwards, leading him to send the aircraft unintentionally into a dive at full power.
I read the same explanation in Aviation Week's review of the final crash report Backfire. Although, the report did state that there were a number of contributing factors (in addition to the above).
I dumped at the gybe mark in strong winds when I looked up at a Porter Q400 on finals. Can't stop spotting.