We report in situ observation of dislocation motion in the active area of forward biased 4H SiC PIN devices by employing an optical emission microscopy technique. Low current density and temperature elevation required to trigger this motion imply that a recombination-enhanced mechanism is involved. The glide of the partial dislocation loops is restrained by defects fixed in the epilayer, which then act as nucleation sites for the bright-line/dark-triangle formation. The stress-generated features are interpreted as multiple stacking faults spreading throughout the whole base region and nucleating in the vicinity of built-in defects and process-induced structural deficiencies. The detrimental impact of the planar defects on the carrier transport properties was directly confirmed by time-resolved electroluminescence measurements. Thus, the observed degradation of the SiC device performance is a direct consequence of gradually increasing density of electric stress induced defects in the active area.