We analyze the dynamics of broad-area mid-infrared quantum cascade lasers (QCLs). We show the possibility of the coherent coupling of several transverse modes which results in several interesting effects including frequency and phase locking between transverse modes, bistability, and beam steering. We present an analytical model for the modal dynamics and its numerical analysis. Effects of amplitude and phase fluctuations on the modal stability are explored. We compare our theoretical results with our experimental measurements of buried heterostructure QCLs.