From what I know, I don't believe any mental illness itself causes premature death except for the fact that some may drive a person to suicide or their lifespan might be reduced due to bad habits or not taking care of yourself. Some psych meds can cause diabetes or heart problems or even sudden death which can of course lead to premature death but this isn't caused by the mental illness itself.
I think most early deaths due to bipolar are from bad habits, suicide, and long term effects of medication (diabetes, heart disease, etc.). I don't think the disorder itself causes early death, just the effects it has on us and the ways we treat it (prescribed and not prescribed).
Suicide is probably the only direct cause of death in my opinion.
That is of interest and I will look it up further but bipolar is not related to that. Bipolar is probably a dysfunction in the limbic system from what I know. People with bipolar would have a normal lifespan but the impact of the depressive episodes causing suicidal ideations and the manic episodes causing people to have psychomoter agitation which makes them physically sped up (before medication existed people in a manic state would not sleep at all and that would eventually be fatal) and make poor decisions or eat an unhealthy diet or exhibit risk taking behavior. Also and this can be managed with a proper diet, the atypical antipsychotics can in some people cause diabetes though the next generation antipsychotics such as the one I am under clinical study for that is in Phase II FDA study (google "Dr. Javitt, glycine", the medications in development are the NMDA receptor modulates) do not cause diabetes or tardive dyskinesia and promote a fuller recovery so as treatment improves so will people's recovery rates and in general their life span and overall health mentally and physically will improve.