  We suggest that the hysteretic cycle of black hole state transitions arises
from two established properties of accretion disks: the increase in turbulent
stress in disks threaded by a net magnetic field and the ability of thick (but
not thin) disks to advect such a field radially. During quiescence, magnetic
field loops are generated by the magnetorotational instability at the interface
between the inner hot flow and outer thin disk. Vertical flux is advected into
and accumulates stochastically within the inner flow, where it stimulates the
turbulence so that $\alpha \sim 1$. The transition to a geometrically thin
inner disk occurs when $L \sim \alpha^2 L_{\rm Edd} \sim L_{\rm Edd}$, and the
first "thin" disk to form is itself moderately thick, strongly magnetized, and
able to advect field inward. These properties favor episodic jet production. As
the accretion rate declines magnetic flux escapes, $\alpha$ decreases to
$\alpha \sim 0.01 - 0.1$, and a hot inner flow is not re-established until $L
\ll L_{\rm Edd}$. We discuss possible observational consequences of our
scenario.
