In the mechanism of Hofmann rearrangement, one step involves migration of $R-$ group from carbon to nitrogen as shown:enter image description here This reaction proceeds even if the migratory aptitude of the $R$ group is poor (like in the case of ethanamide). What motivates the migration of $R$ group and makes the reaction proceed?

P.S.: I think the reason cannot be the negative charge on oxygen since it already delocalized by resonance, which shifts the electron cloud partially towards nitrogen, which is in turn partially shifted towards Bromine due to its -I effect. Here's my depiction: enter image description here