With my daughter it was going away to school. We always knew there was a possibility that she would be bi-polar as we have a long history in our family. Although she was admittedly a bit manic in her teen years it wasn't until her first week at school in Rhode Island when she couldn't cope. She spend 5 days in the hospital and the diagnosis came down. Until the med mix was found she spend a good 6 months severely bi-polar, it was painful to watch.
We were told that bi-polar disorder can lay dormant for years and then a sudden traumatic event will trigger it. And it did....