The WRF-Fire coupled atmosphere–fire modelling system was used to investigate atypical wildland fire spread on steep leeward slopes through a series of idealised numerical simulations. The simulations are used to investigate both the leeward flow characteristics, such as flow separation, and the fire spread from an ignition region at the base of the leeward slope. The fire spread was considered under varying fuel type and with atmosphere-fire coupling both enabled and disabled. When atmosphere–fire coupling is enabled and there is a high fuel mass density, the fire spread closely resembles that expected during fire channelling. Specifically, the fire spread is initially dominated by upslope spread to the mountain ridge line at an average rate of 2.0 km h–1, followed by predominantly lateral spread close to the ridge line at a maximum rate of 3.6 km h–1. The intermittent rapid lateral spread occurs when updraft–downdraft interfaces, which are associated with strongly circulating horizontal winds at the mid-flame height, move across the fire perimeter close to the ridge line. The updraft–downdraft interfaces are formed due to an interaction between the strong pyro-convection and the terrain-modified winds. Through these results, a new physical explanation of fire channelling is proposed.