It is generally conceded that some cataclysmic event helped the process along at the KT boundary. However, faunal change was already well under way. The bolide or similar event probably prolonged and intensified the change, and may have weighed heavily in favor of mammals or birds and against non- avian dinosaurs. However, the most likely root cause for the entire process is random chance. Evolution is a non- equilibrium process.
Life is sometimes metastable, or at some steady state.
It is never at equilibrium. So, there is no need to find a cause for instability. The more interesting question is to explain the rest of vertebrate paleontology. Any reasonably complex non-equilibrium process is subject to these sorts of readjustments. The really strange thing is stability.