Overpressure mechanism detection

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As you know, there is some methods to find overpressure generation mechanism like using velocity, density, porosity and velocity-effective stress cross plots.

Here is the link result for one of my research on carbonate formation, (I know the complexity of PPP in carbonate). I have used velocity-density cross plot and separate the depth intervals by 100 meters: http://i59.tinypic.com/w7243t.png

The top of overpressure is at 2800 meters. As you see in the picture, the velocity-density cross plot below this depth (2800 m) deviates from the NCT to back at above the normal trend.
Which mechanism do you think cause this deviation? Disequilibrium compaction, unloading, chemical diagenesis, or other?