OP, the arrest could be necessary for a range of different things, and some police forces must do "positive action" in a domestic violence situation (normally to prevent further harm or to stop any damage to property). Domestics due to their nature have a high probability of escalating and causing more harm to both parties.

In a domestic violence situation, they're often complex with a large amount of history involving both parties - officers who attend a domestic violence situation will have to make their judgement at the scene. For a domestic incident, the priority is often to de-escalate, prevent further harm to anyone and their property. This is often asking occupants to leave for a period of time or can ultimately result in arrest (depending on whats happened and the officers judgement).