Your accident most certainly could be a trigger. Triggers dont have to be lifechanging events (although the accident you were in sounded pretty severe). After the mood episode is triggered, even when the trigger is resolved it may not have any influence on your current mood.

My last trigger into depression was work stress, before that I was stable for a long time. I was annoyed that stress (even though not severe in my opinion) affected me so much. I am trying to accept that I cannot handle stress at all, and need to be very very aware of my stress levels.