What kind of emotion would make your belly 立つ (stand up)? A pretty angry one, I bet. You'd really have to get mad for your belly to go to all that trouble.

腹が立つ is an everyday expression used when someone upsets you, with the image of your anger and irritation rising up from your gut. Note that the particle が often drops, especially in casual conversation.

This word is just 腹 and 立つ with a が in the middle. Maybe the が is separating them because they're angry and trying to fight each other.

Note that the が isn't always there, though — it's sometimes dropped in casual conversation. There's also an alternate が-less version of the word pronounced はらだつ that you may encounter in more formal settings or writing.