It does sound like your BMS sensed a high voltage event (HVE) and opened a contactor.
We really need a lot more info on this event to be of any help to you:
> What are the absorption and float values for your alternator
regulators and what is the high voltage cutout (HVC) for your BMS?
> Does the BMS respond to a battery
pack HVE or a cell level HVE?
> Does the BMS open a relay in the power to the alternator
regulators prior to opening a contactor to ensure that the alternator output diodes are not over stressed?