With discussion that took place at The Rocketry Forum
http://www.rocketryforum.com/showthread.php?52053-Non-Laminar-Airflow
It seems it could from the heating of the clear coat due to high velocities, or as mentioned within the forum.
When the rocket was entering the ground at a really high velocity, it began to be heated by friction which then made the clear coat absorb the dirt.
More details to come.