  We study the mechanism responsible for the onset of instabilities in a chiral
phase transition at nonzero temperature and baryon chemical potential. As a
low-energy effective model, we consider an expanding relativistic plasma of
quarks coupled to a chiral field, and obtain a phenomenological chiral
hydrodynamics from a variational principle. Studying the dispersion relation
for small fluctuations around equilibrium, we identify the role played by
chiral waves and pressure waves in the generation of instabilities. We show
that pressure modes become unstable earlier than chiral modes.
