Despite being the best studied red supergiant star in our Galaxy, the physics behind the photometric variability and mass loss of Betelgeuse is poorly understood. Moreover, recently the star has experienced an unusual fading with its visual magnitude reaching a historical minimum. The nature of this event was investigated by several studies where mechanisms like episodic mass loss and presence of dark spots in the photosphere were invoked. We aim at relating the atmospheric dynamics of Betelgeuse to its photometric variability, with the main focus on the dimming event. We use the tomographic method which allows us to probe different depths in the stellar atmosphere and to recover the corresponding disk-averaged velocity field. The method is applied to a series of high-resolution HERMES observations of Betelgeuse. Variations in the velocity field are then compared with photometric and spectroscopic variations. The tomographic method reveals that the succession of two shocks along our line-of-sight (in February 2018 and January 2019), the second one amplifying the effect of the first one, combined with underlying convection or/and outward motion present at this phase of the 400 d pulsation cycle, produced a rapid expansion of a portion of the atmosphere of Betelgeuse and an outflow between October 2019 and February 2020. This resulted in a sudden increase of molecular opacity in the cooler upper atmosphere of Betelgeuse and, thus, in the observed unusual decrease of the star's brightness.