So how do you explain such a phenomenon?
Picture close alternate realities, stacked tight together in a pile. Now, drop a bowling ball in the center. Each shard is at a different level that it was, in line with shards from different panes of glass, or even between panes, and in each of these levels, time is flowing at a different rate. So each shard, or bubble, actually existed in a parallel universe, where a different amount of time had passed in terms of revolutions around the sun, etc. The boundaries of the bubbles, while they existed (and some may, still!), protected the inhabitants of this shard from crossing the fractured surface of the shard. As the planets and suns come back into incidental alignment in their different panes of glass again, the shards can snap back into their original planes, drawn by microcosmic telltales embedded in matter itself, and the bubbles disperse.
That realignment, or 'popping of the time bubbles,' would come with violent weather as the membrane between the area and the outside world shattered, and in cases where physical faults may be, earthquakes and volcanic eruption would occur.
The plates of glass analogy is not perfect, but it is a useful visualization for the process, and helps explain why time within a bubble is not stretched.
What did the boundary/bubble edge look like when it was intact?
The actual bubble boundary would be a slipzone between two speeds of time. Proximity to it would make most non-temporal-stable people (basically everyone but the Northerners) ill at some distance (as much as a mile or two), nauseous, dizzy, sort of altered, as if inebriated, with symptoms increasing drastically as you got closer. The sheer unpleasantness of it would drive off most people from approaching it. Visually, I imagine a shimmery sort of haze or thick fog.
The Northerners would have seen Others clustered along this boundary, as well, and kept their distance for that reason, even if their temporal stability saved them some of the other side effects.