This means that you are deforming the element faster than a stress wave can propogate across it.   In effect - you are breaking the sound barrier.

Most often the cause for this is inconsistant units.  Be sure that your material density is correct.  Remember that the density of steel is NOT .283lbs/in^3 because that is a unit of force density in a volume.  If your modulus is in PSI then your density must be in lbf-in^2/s^4.

It is also possible you have a highly distorted mesh and you have a sliver element which is flattening out or worse.

Also, if you are using automatic mass scaling, you may be scaling too much and asking ABAQUS to increase the density of the material too far.  Take a look at your .sta file for the percentage mass change if you are using this option.

In ABAQUS/Explicit 6.4 there is an option called "distortion control".   This option puts a hard stop on the element, preventing them from exceeding the deformation ratio.  This may solve your symtom, but I think it would be better to find out the underlaying cause and solve that instead.

I hope this helps.

-KF9RI