Whats that if F°? Like right around 0?
-20C = -4F
-25C = -13F
Definitely cold, but not the coldest night of the year in those parts. Of course, wind chill would make it worse. In my experience, at those temps, you might run outside for a few minutes without proper clothing if you expected to be able to come back & warm up immediately. I think I mentioned on the comments page that I remember having to evacuate my college dorm in those temps when someone burnt the popcorn & the fire alarms all went off. We had on shoes and sweaters, but no coats. It was cold enough to make your nose hairs start to freeze, but we weren't in agony or anything. And, obviously, we were back inside in 30 min. or so.
It's a good question, why they'd go all the way down the hill (nearly a mile!) rather than just partway.
Assuming that they were able to think clearly & act accordingly (i.e. putting aside psychosis, hallucinations, injuries, or villains forcing them), they must have thought they were safer in the forest than on the open hillside. This implies either that they thought the problem at the tent would not be resolved in a matter of minutes, or that the danger was so large that the hikers felt they must go nearly a mile away to be safe from it.
One important question is why their footprints remained long enough to be found by the search crew--a situation which, I understand, hasn't been successfully replicated. In the photos, you can see that the footprints actually seem to be raised, and the snow around them blown away. This would happen if the snow under their feet had been slightly warmed, packed, & then refrozen so as to make it a bit harder than the surrounding snow.
Ball lightning theory includes the idea that it was the heat from the "lightning" (or whatever it technically is) which slightly warmed the snow enough to freeze the footprints in place. If this were so, then it might be reason enough to head for the treeline: to escape the larger area which the lightning might impact, indicated by the radius of the heat.