If you've seen actual footage of buildings collapsing, they usually fall in on themselves because the structure no longer supports their own weight. We will simulate this by animating the building image to sink behind the other buildings. This is a bit of a cheat, but the smoke and debris elements we will add later will cover up the fact that we're simply faking it.
Keyframe the position of the Building Image to slowly sink downwards after the explosions hit. I also gave the building just a tiny bit of rotation so it appears as if the building is leaning as it collapses.
If you now return to the parent composition, the Skyline Matte is applied to all layers within our Building Comp and elements will sit behind the foreground buildings again. You should now have a cool effect of a number of explosions going off on the building, followed by the building slowly collapsing behind the other structures.