In an investigation of the dynamics of coupled fluid-combustion-buoyancy driven problems, an idealised model formulation is used to investigate the role of buoyancy and heat release in an evolving boundary layer, with particular emphasis on examining underlying fluid dynamics to explain observed phenomena arising in forest fire propagation. The role played by the Froude number and background ambient wind in affecting various characteristics observed in propagating fires is addressed. A simplified flow situation is modeled in which controlled amounts of volumetric heat is injected into the system. By varying the strength of the heat source and the ambient winds the study examines the impact of these variables on the flow dynamics and the consequential development of instability waves that may provide insight into environmental conditions that contribute to erratic fire behaviour. Analysis suggests that there are two routes to fire behaviour and destabilisation, with Froude number playing a crucial role in the boundary layer development and consequential instability of the convecting flow. In low Froude number situations, the mechanism of fire induced local winds may arise as a product of heat release and buoyancy, which induces favourable pressure gradients accelerating local winds to above the ambient. The analysis shows that a massive destabilisation takes place and that stationary, zero streamwise and nonzero spanwise wavenumber, viscous disturbances have the highest growth rates. These stationary spanwise vortex disturbances, commonly referred to as pure vortex longitudinal 'roll cell' modes, might well be linked to cross roll features that have been identified in some recent fire related numerical simulations. The simple model shows that a key requirement in breakup of the fireline, is a low enough ambient wind. In high Froude number cases, the most probable factor is the sensitivity of the boundary layer to separate and lift off the surface; this being caused by massive updrafts of buoyant air. The relatively weak nature of instabilities in this regime suggests that the convecting fireline development would otherwise be well behaved.