Google Home called 911 and the operator heard a confrontation in the background
It's actually soon to be mandatory for new cars to automatically dial 911 using their onboard GSM when they detect a collision.
They were house sitting. It's not a stretch to think that homeowners had other smart devices to listen in and see what's going on remotely, and call police if necessary. Nest cam possibly? It works with Google Home.
Also, there was a daughter (no mention of age) who may have been able to trigger an alarm system or call herself (though a traditional call negates the "smart device" claim by the sheriff's department).
I'm concerned if smart devices will start intervening with human arguments. When they're necessary, it's indeed a lifesaver. When they're not it can really break up otherwise healthy homes and cause a lot of stress.
http://ncadv.org/learn-more/statistics
It's also a notorious blind spot for law enforcement – even when restraining orders are issued, they're nearly unenforceable, and once the aggressor is in your house you're a sitting duck.
Ambient audio-UI devices could be the biggest technological break ever in monitoring for the presence and emotional state of the subject of a restraining order. It's not obvious like a security camera or alarm system, and opt-in to a monitoring app would be invisible.