Numerical simulations are used here to obtain further understanding on the flashback mechanism of a technically premixed hydrogen flame operated in lean conditions. Recent work from the authors (Mira et al., 2020) showed that the hydrogen momentum strongly influences the flame dynamics and plays a fundamental role on the stability limits of the combustor. The axial injection influences the vortex breakdown position and therefore, the propensity of the burner to produce flashback. This work is an extension of our previous work where we include a detailed description of the mixing process and the influence of equivalence ratio fluctuations and heat loss on the flame dynamics.