SELCAL apparently failed to wake them.
The loopy track after overflying MSP was dictated by ATC to be sure the pilots were flying the plane.
Roused only by having other flights contact them on the Denver frequencies.
Source:
Radio Calls Went Unanswered - WSJ.com
And the flight was met at the gate by police, and passengers not allowed to disembark until apparently police were satisfied that nothing untoward from a security standpoint had happened. Hhmmmm, I recall that UA 842's diversion into Miami was also met by the police.