Domestics are dangerous, usually rated among the top three most dangerous things cops have to do..
So first and foremost you have very unstable emotions from the start. An officer is essentially entering a fight in the middle, and not just any fight, a fight between people who share some kind of bond so the emotions in that fight are heightened even more. When people are emotional they don’t make the smart decision. They see the police as intruding on their personal business and will lash out at them or their partner. Which is doubly likely as regular domestic abusers often are abusive for feelings of personal power and control, and police are a direct threat to that control.
Whilst that couple are at odds with each other, they could/can easily turn on the respective officer in a bid to protect one another
Next is just the sheer amount of unknowns. Most of these calls are within a home. A place the officer is ultimately unfamiliar with and may not know if anyone inside has access to firearms or is actively armed. The home offers concealment and cover to an offender whom decides to attack the police.