Ah.. Looking at that, It's the overhang that extends outside the 5x5 Stability is infinite when measured from bedrock- But any open space will begin calculations.
Concrete has a weight of 10, and a horizontal support of 120, meaning it can support up to at most 12 blocks of concrete(Or some combination of other weighted blocks totaling to 120 or less). This measurement begins at the 5x5 area, because the overhang is not supported from beneath. So you have 1, 2, 3, 4, 5, 6, 7, 8, 10 blocks of concrete in one half of the loop on each corner, with the 11th being the 'center' block of each archway.
Then you remove the support from the corner underneath it, and suddenly it's having to stretch several extra blocks for support, beginning the calculation down at ground level now, which pushes it to 13+ total blocks supported, exceeding the support and causing a collapse. Collapse then propagates several times to alleviate the stress (After all, when something collaps...
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