Concentrated emulsions flowing through channels of varying widths are omnipresent in daily life, from dispensing mayonnaise in our kitchens to large-scale industrial processing of food, pharmaceuticals, etc. Local changes in channel geometry affect the stability of emulsions over length scales far beyond the droplet magnitude, for example through propagation of coalescence events called a coalescence avalanche. The underlying mechanisms are not well understood. In this work, we investigated the stability of concentrated emulsions flowing through microchannels featuring a constriction. We found that in this model geometry, the acceleration of the droplets induced near the entrance of the constriction triggers a coalescence event between the leading and the trailing droplet, but only above a critical droplet velocity. This separation-induced coalescence event, in turn, was found to trigger a coalescence avalanche in the upstream direction. Analysis of the flow behavior through particle image velocimetry and particle tracking velocimetry revealed that the propagation also follows a separation-induced coalescence mechanism, due to the retraction of the interface of the trailing droplet upon coalescence and the corresponding acceleration of the liquid inside the coalesced fluid thread. The constriction ratio was found to enhance the coalescence occurrence but did not affect the speed of coalescence propagation.