There are a lot of triggers and everyone is different. I think stress is the most common one. Then there is: other illness, medications, street drugs, alcohol, lack of sleep (therapist says that's the second most common cause), weight loss supplements. That's all I can think of for now.

For me, I tend to run high in mood most of the time. I'm very susceptible to hypo/mania. Sleep disturbance can shoot me either way. Weight loss supplements make me hypomanic. Alcohol causes depression. Oh ya caffeine! Antidepressants cause mania. Stress is the #1 reason to why I go up and down.