Rocket-based Combined-Cycle (RBCC) engines offer a promise for efficient and flexible propulsion over a large Mach number range by combining rocket and ramjet/scramjet technology. To achieve this, an RBCC engine uses four different modes of operation: rocket, ramjet, scramjet and dual-mode. During operation, the engine must make the transition from subsonic to supersonic combustion, i.e., ramjet to scramjet mode. The objective of this study is to gain physical insight into the ramjet-scramjet-ramjet mode transition by elucidating the underlying mechanics. Numerical simulations with chemical reactions have been performed for the transient flowfields of a two-dimensional RBCC combustor by using an unsteady Reynolds-Averaged Navier-Stokes solver. Mode transition is effected by changing the flow rate of the secondary hydrogen fuel injectors installed on the top and bottom walls of the combustor. A parametric study was conducted to investigate the characteristic and behavior of RBCC combustion in mode transition. The results indicated that transition is affected considerably by the presence and development of flow separation and pseudo-shock structures near fuel injectors. The complex effects of aerodynamic and aerothermal interactions on the transient flowfields and performance, along with a hysteresis observed between the scramjet-to-ramjet and ramjet-to-scramjet transition processes.