Pitting of direct filling gold restoration usually
results from:
The gold is first pressed into place by hand, then a condenser of suitable size is used to begin malleting in the center of the mass (often this is done while this first increment is held in position with a holding instrument). Each succeeding step of the condenser overlaps (by half) the previous one as the condenser is moved toward the periphery. The gold moves under the nib face of the condenser, effecting compaction as malleting proceeds.
The most efficient compaction occurs directly under the nib face. Some compaction also occurs by lateral movement of the gold against surrounding preparation walls. The result of compaction is to remove most of the void space from within each increment of gold, to compact the gold into line and point angles and against walls, and to attach it to any previously placed gold via the process of cohesion. The line of force is important when any gold is compacted. The line of force is the direction through which the force is delivered (i.e., the direction in which the condenser is aimed). 
Thus improper stepping of the condenser point leads to voids (pitting) of direct gold restorations.
Ref: Sturdevant operative dentistry 7 ed page no e72
Using contaminated gold foil
Poor finished preparation
Improper stepping of condenser point
Using small condenser point