Forgot to point this out - notice that the lime-green part of the SPX chart (top) looks uncannily similar to the ight-blue portion of the Gold chart (bottom).
Notice also that the correlation histogram for each area is a mirror image of the other. This is true from a visual standpoint and from the actual readings, which swing from near-perfect inverses to near-perfect positives.
While I can't say for sure, symmetry to this extent is uncommon and thus worth a hypothesis. The other factors mentioned in the chart, however, are commonly seen aspects involved with the (un)pairing of correlated assets.