Mixed halide (Br/Cl) perovskite nanocrystals (NCs) that represent an advanced blue emitter commonly suffer from spectral instability and poor lifespan; notably, the lack of understanding of the failure mechanisms of these devices has restricted the future progress. Here, we determine that the degradation of CsPbBrxCl3-x NCs containing blue light-emitting diodes (LEDs) is due to the combination of two effects. Cl- drift among adjoining NCs under an electric field is found to induce a Cl-deficient material region in multiple NC layers, which dominates the unstable electroluminescence and causes fast degradation in the corresponding devices. In comparison, the monolayer NC devices that feature restricted anion drift pathways exhibit better operational stability; however, excess hole injection is demonstrated to induce irreversible chlorine loss in NCs. Such a process that largely arises from electrochemical oxidation of Cl- initiates a mild device failure in operation of tens of minutes. Through revealing these mechanisms, we modulate the devices' construction and operational conditions to achieve a longer lifespan.