From colloidal particles to pedestrians in a crowded city, the “jamming” phenomena, characterized by the kinematic arrest, is universal. Studies to understand the jamming state and to resolve the undesired jamming in various situations have recently gained a great deal of attention. Interestingly, cells in our body can also experience the jamming, and the unjamming can lead to pathologically relevant consequences. Physiologically, cells in a collective entity exhibit reminiscent characteristics of the physical jamming phenomenon, including various factors like interrelation between neighboring cells and the cell-substrate interaction. While this peculiar state in the collective cells has been reported in various biological studies, its driving mechanisms and contributing factors have not been revealed clearly. In this study, we investigated the jamming phenomena inducing a spontaneous jamming-unjamming transition in the initially confined monolayer of MCF-10A, normal human breast gland cells, through the simple wound healing assay. Diverse phenomenological analyses, from the morphology of cells to the stress distribution inside the cells were performed to find critical cues in the unjamming event of cells. Besides, we could compare the two biochemically different conditions, which were initially undistinguishable in the jammed state, by unraveling phenotypic features in the unjamming transition. These results demonstrate that the approach of this study can expand the boundaries of the jamming studies to general cellular jamming-unjamming phenomena reflecting more practical physiological events.