Anger outbursts can be the result of underlying stress and frustration. You could be pushing aside stressful experiences, situations where you can't do anything to fix the problem causing the stress. Letting them build up inside and you unload your anger at your loved one.

For example you could be daily frustrated at side effects of your meds, stuff you can't do because of your condition or things that apply to none bipolars as well. If you push those stuff inside and try to ignore them the stress they cause doesn't just go away like that, it can linger and build. Or maybe a clerk in a store disrespects you somehow, you might want lash out, but decide not to in front of so many people. That builds too.

A basic example that applies to many people with anger problems is frustration at work and with their boss or customers, an employee can't lash out at either no matter how justified they feel so they let those emotions stew inside. They think its disappeared or the feelings will just go away, only to find out it is lashed out at loved ones when they get home. If a guy doesn't have work, that causes lots of stress for many guys.

Every person, not just Bipolar have stressors. Everyone at a time has lash out at their partner to relieve stress, even when the partner had nothing to do with the cause of it.

If you don't find a constructive way to deal with stress, it can explode on you. Doing meditation can definitely help with that, exercise also really helps. Eating well makes a big difference. You can never eliminate all stress causing events and you can't deal with all them when they come up.

You could try talking to your partner about stuff that bugs you daily, stuff that you just regularly push aside.