Actually, they're not necessarily training.
They drive tight like that on purpose to keep cars from passing the rear plow, which is on the right when you catch up to it.
Once people got passed it, they would sometimes lose control getting back to the right hand lane over the furrow of the lead plow in the left lane.
And when they did not spin out there, some got caught up in the wing of the lead plow dumping it's furrow to the right.
I mean big mess ups. Tractor trailers getting mangled in the wing of the lead plow.
It's not the Province that decided this. It's the contractor, SNC Lavalin (formerly Brunway) in the north here.
They don't push much snow to the left. They push it all to the right.
Makes it easier for the snow blower after the storms to clear the guard rails and structures because it only needs to operate on one side of the highway.
They deem it safer overall. A bit more frustrating during the storm, such as your case, but sfaer the rest of the time.