The million-dollar question now is how it managed to recover after plummeting 2/3 its altitude, only to do so again shortly after? That shows that it was still operable and could aviate, and opens up many more questions.
I'm not sure if briefly levelling off for about 30 seconds is proof that the plane was "operable and could aviate". It feels more like the pilots briefly got some control back, before the situation worsened again.
Of course really trying to speculate at this early stage on limited data is pointless. We really need to wait for some hard information to come out of the crash investigation.
This is because flightradar24 measures ground speed. A rock dropped from a building has a ground speed of zero. The apparent recovery was due to the fact that the aerodynamics of a 737 in a nosedive force it to pitch up somewhat. But the pilot probably pushed it back down.
Possibly explosive decompression then they go way too fast on the emergency descent so pulling out of the dive caused structural failure leading to the crash.
Of course really trying to speculate at this early stage on limited data is pointless. We really need to wait for some hard information to come out of the crash investigation.