  Accretion-powered X-ray pulsars 4U 1626-67, GX 1+4, and OAO 1657-415 have
recently shown puzzling torque reversals. These reversals are characterized by
short time scales, on the order of days, nearly identical spin-up and spin-down
rates, and very small changes in X-ray luminosity. We propose that this
phenomenon is the result of sudden dynamical changes in the accretion disks
triggered by a gradual variation of mass accretion rates. These sudden torque
reversals may occur at a critical accretion rate $\sim$
$10^{15}-10^{16}g~s^{-1}$ when the system makes a transition from (to) a
primarily Keplerian flow to (from) a substantially sub-Keplerian, radial
advective flow in the inner disk. For systems near spin equilibrium, the
spin-up torques in the Keplerian state are slightly larger than the spin-down
torques in the advective state, in agreement with observation. The abrupt
reversals could be a signature of pulsar systems near spin equilibrium with the
mass accretion rates modulated on a time scale of a year or longer near the
critical accretion rate. It is interesting that cataclysmic variables and black
hole soft X-ray transients change their X-ray emission properties at accretion
rates similar to the pulsars' critical rate. We speculate that the dynamical
change in pulsar systems shares a common physical origin with white dwarf and
black hole accretion disk systems.
