I have a lot of experience shooting in cold weather down to minus 30, (I'm from Manitoba too) but usually-20. I've never seen any of the problems put forward on this thread relating to moisture, although I would hesitate to discount completely. When hunting I routinely tape the muzzle to keep snow or mud out, in case of tripping. I shoot through the tape.
But to the issue, by the shape of the case, and the taper produced, it seems less likely it was an annealing problem. It also looks unlikely it was an (ordinary) overpressure load that could happen with a few grains. I've had that happen, more than once I'm sorry to say, and the unbelted .243 case looked to have a belt on it. The case pictured seems to have expanded evenly from shoulder to base. Looks like pressures well beyond even factory proof loads. This really looks like a detonation.
I'm pretty confident the loading practice described was good enough to avoid loading a bad round, so if you accept that, there are only a couple of things I can think of.
One is a bad primer from the factory. I've never had a problem with Federal 215's even with compressed charges of H870 in a .300 Weatherby in temperature down to minus 35 which is what it was when I shot my first elk. But I think it could happen with any manufacturer.
The second is a bad primer again, but caused by the user. I loaded for a number of years and was occasionally plagued with dud rounds. Finally I read or was told that some people can not directly touch a primer as their skin can give off oils that can kill or ****** a primer. That was me, I guess, as I never had another failure in a rifle after I quit handling them. I'm guessing that they might be damaged in the seating process or a high primer might not ignite properly either.
I've had an experience or two that I could attribute to high primers in pistols when I failed to notice a change in my press over time. These can cause squibs, and that could well lead to a detonation in a big case.
The only other thing is an underloaded case which has historically been a cause of catastrophic failures with slow burning powders, but I've already said I'm confident that was not the cause, as the loading procedures followed seem to have been excellent.
In any event I'll be following this thread. I am comfortable with my assessment but am interested to see what else comes up.
McDoone