How on earth did someone calculate a descent rate of over 30000 fpm? That's ~550 kph, vertically. Hardly a believable figure...
As other have pointed, the initial descent (with corresponding speed increase) seems legitimate. It could perhaps point to an initial problem/abnormal indication being detected and reasonably corrected given the controlled climb to 5000-ish ft with ~300 kts. The flight at altitudes near 5000 ft could point to the initial problem being troubleshooted (I'll play the UAS scenario); minor deviations from altitude target (which seem to be 5000 ft) which are consistent with speed indications: ie. altitude goes down, speed goes up; altitude goes up, speed goes down. So maybe they really were applying a UAS checklist and flying pitch/thrust figures and trying to identify the culprit(s). Then something goes terribly wrong, given the fact they lost +2500 ft (~5000 to 2800, which is the last recorded value in some of the graphs posted above) in just under 30 seconds...