This paper analyzes the harmonic instability due to interactions among power converters and negative incremental impedance of load converters in a dual active bridge (DAB) converter enabled DC microgrid. Based on derived system impedances, it is found that the challenge of a DAB converter enabled microgrid stability is low frequency terminal behaviors of integrated units, which is different from other converters powered DC microgrids. At this frequency range, tightly regulated load converter exhibits constant power load (CPL) behavior even with low control bandwidth, which degrades system stability by aggravating interactions among power converters. In addition, the deployed power management strategy enables mode transition of energy storage units to achieve high reliability, which further complicates system impedance characteristics. To solve these issues, global minor loop gain (GMLG) and unified stability metric (USM) are derived to analyze system stability under different operation mode. A reduced-order low frequency model is developed to provide an insightful view of resonance mechanism. Based on the proposed model, the effect of power converter interactions and constant power load behavior are analyzed coordinately. Finally, an effective impedance shaping technique is proposed to improve system stability by eliminating resonance path in the reduced-order low frequency model. Experimental results and more simulation results will be provided in the final paper.