Causes and effects.
It isn't really caused by anything, unless by "cause" we mean Aristotle's "material cause", which would be: fucked-up communication between neurons in specific brain centers. But episodes can be triggered, which would be Aristotle's "efficient cause". Pretty fucking efficiently triggered sometimes, let me tell you.
My most prevalent trigger is disappointment-leading-to-loneliness. This is where we should be doing philosophy in German, 'cos we need those hyphenated expressions. Depression results in a lack of emotional reserves, where you're brittle all the time. Things go wrong: where someone else would cruise past that, for me it can be as though the ground falls from underneath, there's actually literally a sinking feeling in my brain similar to what you feel physically when descending in an elevator. And then I'm fucked, usually until several hours sleep recharges things.
This is in turn the origin of codependent relationships. It's better to be in an unhealthy dynamic than to be alone. Worsened, basically immediately, because you become so desperate for the relationship that you take on the other person's crazyass troubles. At which you're in no possible position to succeed.