The single strongest correlate is flooded vegetation, and it runs the opposite way to what you might guess. Tiles dominated by flooded vegetation come out with the lowest reconstruction errors of any land-cover class, not the highest.
Evidence
correlational
Key metric
Spearman rho = -0.32 for flooded vegetation
Caveat
The authors offer the bimodal optical and SAR signature of inundated wetlands as a plausible explanation, not a tested one.