CavVet,
It could have been something that started up in thier jurisdiction. As a cop, I can tell you that the last thing I want to do is make stops outside of my area, but if people are extra stupid, sometimes I have no choice.
A quick example. A guy was punching his very pregnant girlfriend in the stomach as I was driving by, for some strange reason, I felt obligated to deal with this.
Another example. On the way home in a marked car, I see a car load of kids hit a road flagger in the face as they drive by. Again, I felt obligated to do something.
If I deal with something outside of my area, it is more paperwork and aggravation for me, but every once in awhile, it needs to be done.
Hopefully this makes a little sense. I'm not big into the police getting involved in peoples lives, nor am I a fan of big government, but sometimes things need police involvement.