  We present a study of the model spin-glass LiHo$_{0.5}$Er$_{0.5}$F$_4$ using
simultaneous AC susceptibility, magnetization and magnetocaloric effect
measurements along with small angle neutron scattering (SANS) at sub-Kelvin
temperatures. All measured bulk quantities reveal hysteretic behavior when the
field is applied along the crystallographic c axis. Furthermore avalanche-like
relaxation is observed in a static field after ramping from the
zero-field-cooled state up to $200 - 300$ Oe. SANS measurements are employed to
track the microscopic spin reconfiguration throughout both the hysteresis loop
and the related relaxation. Comparing the SANS data to inhomogeneous mean-field
calculations performed on a box of one million unit cells provides a real-space
picture of the spin configuration. We discover that the avalanche is being
driven by released Zeeman energy, which heats the sample and creates positive
feedback, continuing the avalanche. The combination of SANS and mean-field
simulations reveal that the conventional distribution of cluster sizes is
replaced by one with a depletion of intermediate cluster sizes for much of the
hysteresis loop.
