Even though BMW probably considers it proprietary, you should ask for the Airbag Sensor/Event Recorder download in graphical and text form. The data should indicate the particular threshold that was exceeded to have a deployment event. You may have had an unusual lateral movement or something short and sharp that initiated the deployment sequence even though it seemed nothing radical occurred in the vehicle itself. Airbags very very rarely "go off" by themselves, in fact I've never seen a pre-event deployment.

Whatever the case, the trigger event should be in the event recorder flash memory and available through a download program - which I'm sure BMW doesn't share! Good luck.