I'm pretty sure the husband/officer is a member of a different police force.. In Canada, we have a sometimes complicated system, but I'm sure parralels can be drawn to other countries..
We have the RCMP, or Royal Canadian Mounted Police.. they are federal, juridiction is (technically) every square inch of the country.. (contrary to popular image, only a small number wear the red serge uniform and perform in the musical ride your mind's eye is probably showing you).
Then we have provincial police, in this case the Ontario Provincial Police, or OPP.. thier juridiction is limited to the province itself. They are often the police of smaller towns and "the spaces in between".. Some provinces and territories may not have thier own police, and rely on the RCMP to fill the void.
There are regional forces, like the York Regional force that the husband is/was employed by, responsible for the city/township/county they're named for (York, a suburb of Toronto, in this case)..
Then we have 'local' police, like the Toronto (or Ottawa, or London, or Windsor, etc) Police dept.
The standard proceedure when an officer is being investigated is to bring in someone from another force to do it.
Odds are that when the officer arrived on scene he had no idea who the involved driver's husband was, and if he had any association with him he would have been required to hand the case over to another officer or agency for investigation due to (potential) conflict of interest.
Hope that clears a little confusion.