  We numerically elucidate the microscopic mechanisms controlling the
relaxation dynamics of a three-dimensional lattice glass model that has static
properties compatible with the approach to a random first-order transition. At
low temperatures, the relaxation is triggered by a small population of
particles with low-energy barriers forming mobile clusters. These emerging
quasiparticles act as facilitating defects responsible for the spatially
heterogeneous dynamics of the system, whose characteristic lengthscales remain
strongly coupled to thermodynamic fluctuations. We compare our findings both
with existing theoretical models and atomistic simulations of glass-formers.
