SLFs begin to chime in with their inept theorizing
As a long time lurker I won't argue with all your observations on thread arcs; but as an SLF with long-standing interest in aviation accident forensics* I'll chime in anyway.
Could this be a case of running afoul of a mountain rotor while sightseeing, a'la BOAC 911 and Mt. Fuji in the 60's?
Back to lurking....
* As a wee lad I was always fascinated by the forensic details teased out of bent metal and recorders as reported in the AvWeeks that Dad would bring home from his IT (or whatever they called it back then) job with EAL.