A durable catalyst layer (CL) is essential for the successful commercialization of anion exchange membrane water electrolyzers (AEMWEs). However, AEMWEs often exhibit a large performance drop in the early stage of operation, which has not been clearly understood. Herein, we unravel the root cause of the initial degradation for a typical anode CL composed of a quaternized polycabozole-trimethyl amine (QPC-TMA) anion exchange ion-omer and IrO2 nanoparticles. Post-mortem and electrochemical analysis reveal that the detachment of the IrO2 nanoparticles from the CL and the interfacial delamination of the CL from the porous transport layer (PTL) surface are responsible for the initial degradation. Nafion (ionomer) as a co-binder prevents the initial degra-dation by increasing the mechanical integrity and interfacial adhesion of the CL, thereby indicating a strong binder effect on the initial degradation. Molecular dynamics and density functional theory calculations elucidate that the binding energy of QPC-TMA on IrO2 or PTL surface is much smaller than that of the Nafion ionomer, because the ionomer-catalyst or ionomer-PTL interaction is sterically hindered for the bulky TMA group. This study motivates structural designing of anion exchange ionomers with strong binding abilities to address the initial degradation problem.