The pitot tube is blocked due to icing.  The windshield may be iced over as well, blocking all outside references even if the airplane does break out into VMC.

The airplane is descending, causing the indicated airspeed to decrease (the difference between the increasing static pressure at lower altitudes and the "high pressure" air trapped in the pitot system is becoming less and less).

The pilot pushes the nose down making the dive steeper and decreasing the ground speed reported by the GPS.  The decreasing ground speed correlates with the decreasing indicated airspeed.  Even the unwinding altimeter could be interpreted as a stalled airplane in a descent by someone focused on a stall as the main problem.

Happened to a 727 years ago....happened to a 757 ( I believe, in South America due to a blocked pitot sytem from a wash job). I will say this again-- turn on the pitot heat before takeoff.

 

Jeff