I've read that it's quite common for someone with bp (and depression, I think) to be able to behave perfectly fine around work colleagues, etc. (a kind of projection of an "I'm fine" persona), but then naturally not to be able to keep that up 100% of the time and/or with people close to them, so those people then get the full force and some (because stuff's been suppressed and then explodes). If this one person you're talking about is someone close to you, that may explain what's happening. If they're not, it could be an anger management issue, or you could actually just be angry with them but not have properly acknowledged / dealt with that yet. Let's face it, though, if you've got bipolar and things are bad, it's not surprising that you'd show that to *someone*; in a sense, it'd be more concerning if you didn't. That's not an excuse for letting someone "have it", but, if you are close to this person, maybe you could explain more to them and deal with it that way.