All kind of processes that contribute to the reworking of soil particules and constituents. It is not only a question of physical factors, bioturbation is involved as well in the incororation of organic matter in deeper horizons.

Moreover the 14C profiles are not always monotonic. The few  studies (see work of Sharpenseel, Pietig, Becker Heidemann) that did high resolution 14C profiles found also some specific shape of the profile around the horizons boundaries

the decline of delta14C values with depth (i.e. C getting older) is due to persistence of old organic matter in deeper soil layers. The main physical factors that are responsible for this persistence in deep soil layers are mostly aggregation and burial - both lead to limitations in oxygen, water, and extracellular enzyme diffusion to decomposing organic substrates ... and the same variables also limit composition, abundance, and activity of microbes that mediate the decomposition processes. 

Off course the physical processes are not the only ones active, chemical organo-mineral associations also play key roles in persistence of old organic matter at depth. 

Thanks for your replies.

Both physical processes and old organic matter together accounts the decline of 14C in the soil.

The physical factors influencing the 14C signature of SOC with depth include aggregation and physico-chemical stabilisation, and conceivably the hydrological properties of soil (as this will affect the translocation rate of the SOC). These will be influenced by the mineralogy and texture of the soil (and in the case of aggregation, also the SOC content). Of course, the depth distribution of SOC is also influenced by external factors such as land-use/vegetation and climate and the time since initial soil formation, and these factors are therefore likely to influence the radiocarbon distribution down the soil profile.