The sources and characteristics of the slow eastward-propagating planetary waves (EPWs) were investigated prior to the major sudden stratospheric warming (SSW) events, as observed by the Microwave Limb Sounder (MLS) and simulated by the National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model (WACCM) with specified dynamics. With a zonal phase speed of ~10 m s^-1, these EPWs appeared as wavenumber-1 and -2 perturbations in the boreal stratosphere and mesosphere prior to SSWs with split and displaced polar vortex. Found near the turning and critical layers, these waves were manifestations of an unstable eastward mesospheric flow. These instability waves were investigated from the perspective of over-reflection. A zonal-mean zonal wind structure with local maxima around the upper polar stratosphere and the subtropical mesosphere was commonly found prior to SSW onset as the result of net forcing by gravity and planetary waves. This structure was largely unstable and provided the wave geometry conducive to over-reflection.