[en] A model is proposed to explain the outbursts observed in compact extragalactic sources. It involves collisions between stars deep in the potential well of large (> approximately 10⁸M solar masses) black holes, which may exist in the nuclei of some active galaxies. The largest outbursts observed have total energy > 10⁵⁴ erg, which is orders of magnitude greater than the observed output of a supernova. High-energy collisions of stars can produce the required energy to explain these large outbursts as well as the more 'normal' variations observed. This model also agrees well with the recent results of Long Baseline Interferometry, which indicate that the outbursts expand relativistically out from a central source rather than the random outburst model