Accepting cityfan's statement that United's investigation is a flight safety investigation, the principal parties (through their proxies) have raised three situations where flight safety could have been compromised:
a.) the purser asserting that providing the captain with the customs declaration document when he sought it would have violated security protocols re: door coverage;
b.) the captain asserting he directly heard the purser threatening to tamper with the crew meal;
c.) the captain asserting that the purser (and perhaps other cabin crew) was insubordinate.
With regard to c.) its unclear why the captain would think the safety of the flight would be more compromised by continuing to its scheduled destination of Chicago, rather than by first diverting to Miami. (cityfan asserts that once the captain announced the diversion, the purser deliberately interfered with the safety of the flight, but that occurs after the diversion decision was made.)
so for the moment, IMO, its a.) vs, b.)
And FWIW, the captain has been a long-time contributor to ALPA's PAC.