Whaco,
During icing conditions the pitot tube, which is the airspeed sensor, can plug up with ice (if the pilot forgets to turn on the pitot heat), and cause the airspeed indicator to read low (when they are really going 150 knots the airspeed indicator might read 60 knots). The pilot, who is already worried about not having enough horsepower to lift all the ice that he expects is building up on his airplane, will look at the airspeed indicator and assume, because of the low indicated airspeed (60 knots) that he is about to enter a stall and correct by putting the nose down to build speed. As the indicated airspeed decreased he had probably already applied full power but as the airspeed dropped his only other option was to push the nose down. Also, if he was in snow or clouds and couldn't see the horizon he may not have realized he was in a steep dive. I'll bet that's what happened.