The reasons for your behavior can be different. Insight does not always lead to change of behavior. Even if you figure out what is triggering it, you can continue to behave this way until you make a commitment to change.
Some speculations as to why you're saying that to him 1/ you're really upset and that is the only way you can express yourself 2/ you're threatening him with the one thing that you have control over (leaving) You may feel somewhat helpless in the relationship otherwise even if you're not aware of it. 3/ This is a habit/it may be something you've observed others do, 4/ you're testing him to see if he really cares about you (although this is no a constructive way to do that)
What may be helpful is to use journaling to help you figure this out and then decide how to work on changing. You can start by making an inventory of whether or not the fights have a specific theme behind (when you say that), what is the feeling behind it? Do you feel betrayed, lonely, misunderstood, ignored, etc. Connect your internal dialog with what you feel. Sometimes you may notice being irrational and then you work on correcting that within you.