Prevailing hypotheses for secondary eyewall formation are examined using datasets from two high-resolution mesoscale numerical model simulations of the long-time evolution of an idealized hurricane vortex in a quiescent tropical environment with constant background rotation. The modeled hurricanes each undergo a secondary eyewall cycle, falsifying a number of hypotheses for secondary eyewall formation due to the idealizations present in the model formulation. A new hypothesis for secondary eyewall formation is proposed and shown to be supported by these high-resolution numerical simulations. The hypothesis requires the existence of a region with moderate horizontal strain deformation and a sufficient low-level radial potential vorticity gradient associated with the primary swirling flow, moist convective potential, and a wind-moisture feedback process at the air-sea interface to form the secondary eyewall. Possible application to the problem of forecasting secondary eyewall events is briefly considered.