It’s always sensitive to speculate without full data/insight.  However, one might “guess” that the initial rapid descent (dive) was to get down to high/normal pressure air, being the cabin would de-pressurize without bleed air???

 

I’m also guessing that the speeds shown are ground speed (raw radar speed)?  Even without any wind, IAS at altitude would be somewhat less (~30% less at 20,000ft).

 

I’m not sure exactly how accurate the radar is, which would effect the accuracy of speed shown/calculated between radar returns, but the “37” @ 4400ft would seem to be a problem (unless there was a huge headwind or perhaps a sharp turn such that 37 was a inferred speed based on straight line distance between radar returns rather than the actual flight path of a 180 turn – i.e. the “calculated” speed of a large circle could be zero).

 

Rick