I story I heard shortly after from a relative who was friends with one of the pilots wife:
Early that morning, he (the pilot) was switched off his normal route BOS to LAX (not totally unusual) and reassigned to a different plane but on the exact same route (seemed weird and why). So this pilot ends up on a different plane behind his usual plane on the taxi way and to be friendly, he tried to hail it on the radio. He got no answer (very unusual) and contacted the tower to report it but they did nothing, even more unusual.
So, if this is true, the plane was either hijacked before take off or was on RC.