If bipolar disorder is involved, it's still a combination of various factors.

Bipolar disorder alone doesn't cause something like this, I'd know because I have it and I can't imagine ever doing something like this, even in my wildest manic episodes, even though I have a pretty serious case of it.

The other factors could be anything. Some examples: trauma, co-morbid mental conditions or just being a shitty person in general. Those are just some examples, like I said, it could be anything and it's likely the other stuff is things we probably couldn't even think of.