If you look at the animated image just above (orange) you will see a feature in the arch that is absolutely critical to stability of the arch. From an architectural point of view, the most important stone is the keystone, the stone that sits at the very top of the arch in the center. This stone carries the highest stress load and when this stone is compromised or removed, the arch will collapse quickly. Often, you will find the keystone is larger than the other stones that form the arch. On this animation, the keystone, a large ice flow of multi-year ice is clearly visible. If we see this floe fracture or if we see the ice around it which is mainly small bits of MYI and FYI which has frozen them in place begin to disintegrate, it is a suggestion of imminent collapse. I would pay particularly close attention to the prominent bulge of ice that protrudes into the open water at the top of the arch on the west side of the keystone. If this gives way, the keystone will lose its support and the arch will collapse IMHO. In that most recent calving, it appears as if this bulge was compromised or chipped away at.
By the way, having ice peel away from the sides, near the base of the arch, increases the load and stress on the keystone.