The EGR is most active at idle and when the engine is cold. Prolonged idle time and short trips are definitely two things that allow it to build up. Tuning with an active EGR will increase the amount of soot buildup. A sticky EGR valve that isn't closing all the way. The PCV is routed into the intake tube. Improper maintenance of the crankcase filter can cause oil to be introduced into the intake, which makes its way to the intake horn, where the EGR is introduced, so you end up with a wet oily film which catches more soot. Those are a few of the ways it can increase soot buildup.
The EGR is intended to reduce NOx emissions by reducing combustion chamber temperature. Exhaust gas has very little oxygen in it, so when it is introduced back into the intake, the butterfly valve located inside the intake horn closes off fresh air, and the amount of oxygen going into the engine is reduced, causing the fire in the combustion chamber to burn at a lower temp.
