Phiggsbroadband
Please excuse me if this has been posted earlier.. But I cannot help thinking that the transponder return cannot go from 35,000ft to 0ft in next to no time.
Even with the nose vertically down it would take 40+ seconds, and more if you take the maximum possible curve into account. Freefall from that altitude would take over 3 minutes, and would be speed limited by the terminal velocity.
The FR24 trace continues several miles beyond the 0ft point. So either someone switched the transponder off, or it had a partial progressive failure.
Short answer, it didn't.
Long answer, that 35k to 0 is someone's less than fantastic software. It you turned off altitude reporting you would get an invalid not zero. And transponders altitude encoding starts 1200 feet below sea level anyway.
So the best guess is someone's code interprets no data as 0 feet.