It's a police procedural term.
It was used in the police reports to which TB referred. One tends to use the terms one encounters when talking about an official report.
"Domestic incident" is a crime, usually violent, involving a domestic relationship. It doesn't define the people involved as married, because frequently they aren't. It doesn't define the nature of the crime because it's still under investigation at the time the term is used.
This particular crime was a double murder, and as we all know now, it was nothing like what the police thought it was initially.