The field of processing materials with magnetic or electric fields and currents is growing steadily as it opens the door to more efficient ways of material treatment and microstructure optimization than were previously unattainable. For example, it has recently been demonstrated that high DC electric current densities can be used to create anisotropic microstructure with elongated ferrite grains in otherwise nanocrystalline iron-carbon thin films. To understand the underlying mechanisms and to answer the question why such behavior has not been reported before, we performed experiments with a wide range of process parameters. The results allow a detailed description of the evolution of the abnormally grown structures and reveal a threshold below which no microstructural changes are observed. The changes are caused by migrating carbides leaving elongated ferrite grains in their wake. The threshold behavior is due to a critical oversaturation of carbon in front of the carbides that has to build up. These considerations will be discussed in detail and could allow the prediction of a similar behavior for different process parameters and material systems. A future goal in this area is to use these considerations to predict process parameters for a similar behavior in a different material system and thus confirm the proposed model.