BOAC and captins, a United pilot posted in another forum around the end of July that the story going around United operations was that the purser had allegedly threatened to tamper with the crew meal, and this pilot suggested that such a threat would make a decision to divert perhaps more understandable.
I surmised that the captain sitting in the cockpit had heard of the threat either second or thirdhand, but cityfan corrected my erroneous presumption by stating the captain knew firsthand. So either the purser told him directly that she was considering/planning doing this, or he was present within earshot when she voiced the threat.
Whether the captain repeated the tampering threat to the authorities at the gate in Miami, or discussed it with dispatch has not been made public. But if he had done either, one would think the purser would not have so quickly been sent on her way by the authorities and United both.