My only guess is that they needed a discrete area to stop. Oak Park Rd may not be that discrete, but it is a long, straight road with very little traffic at night and you can see well in advance if anyone's coming toward you. If they did in fact take the 403 through Brantford and exit at Paris Rd, Oak Park Rd is among the few side streets before you get to Paris. It could have been an impromptu decision to pull over and perhaps move the body into the truck bed. I dunno. IMO there are so many possible explanations and not enough evidence to say one way or the other. They were probably improvising and making decisions on the fly, so it just played out the way it did.
I noticed from the photographs that for a truck box that was supposedly washed out recently, The box of Tims Ram looked pretty muddy to me. That luminal could be high-lighting a long past hunting trip.