Sounds like he may have flown too high too long for his respiratory capacity, and lost consciousness.
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According to FlightAware, he was at 21,000 for about an hour, until 12:00 EDT, then took a little over half an hour to descend to 13,100, where he stayed more or less until beginning a descent shortly after 3pm. FlightAware's last altitude was 4,600 at 3:13, but their record doesn't end until 4:03.
According to the Wikipedia, the service ceiling of the SR22 is only 17,500 ft.