The Rift, by geology, is definitely a caldera, with some indications of being active. But keep in mind that such features rarely, if ever, exist in isolation. Look at the proximity of Vaarnenfell to the Rift area; and the mountain range that connects the regions. Moutains we know are upthrust granite. We could be looking at one of two things: either a plate boundary, with both Rift and Red Mountain being a very weak spot in the crust, or a rebound subsidence, where two plates collided, created the range, then moved apart slightly, so you would actually have a plate fissure, similar to the Reelfoot Rift beneath the Mississippi River channel. If the former, then both Red Mountain and the Rift could be part of Nirn's own 'circle of fire'.....but we'd need to know what features were out in the eastern direction to know that. More likely its a rebound subsidence, as you'll note that there is plenty of evidence of seismic activity in the collapses of solid stone structures, but not a lot of loose scree around the mountain bases to indicate a lot of repetitive quakes. So if it's like the Reelfoot zone, you are looking at lots of minimal quakes, with -major- events every few hundred to few thousand years. The explosion of Red Mountain might have relieved a lot of pressure, and saved that part of Skyrim for a long time. Or it could have destroyed a force balance, and the whole place is living on borrowed time.
We shall see......
From my experience with geological events, and what I have seen happen in the past couple years here on Earth, large events, such as the eruption of Red Mountain could indeed cause other large events. Of course, this is all speculation on my part, but there is some circumstantial evidence to support this. Take 2009 through 2011 for instance. The Earth experienced several large Earth Quakes, two of which occurred on the Pacific Ring of Fire. One in Chile, one in Japan. I believe we can discount the New Zealand quake, because it was no where near the scale of the other two events.
The Chilean Earthquake hit in February 2010, while The Japan quake hit in March 2011. The Chilean Quake reached 8.8 on the Richter Scale, and the Japanese quake hit 8.9. Of course, this could be pure coincidence as to the size, and the closeness in dates of both quakes, but it could also mean a link some where.
If there is a link, it is reasonable to assume that the release of pressure in Red Mountain could also create an instability, and cause a release of Pressure at Bonestrewn Crest.
I was just exploring this region again a few hours ago. I noticed a few things about the area. There are actually a few small mounds that could represent past releases of Magma. Upon further inspection, the rock was indeed black, as if it is ancient cooled Magma. Not only that, prolonged exploration of the area shows that there is 'Rumblings' coming from some where. This was definitely not Dragon caused, as I had already taken care of it when I started that Exploration. As you spend more time in the area, you'll notice low vibrations, as well as the occasional vision shaking. It's best heard with Headphones.