Margin liquidation can turn a smooth price decline into an abrupt loss of market exposure. We study a one-asset absorbing threshold model in which positions are weighted by dollar exposure and forced sales feed back through price impact. The local instability is controlled by the exposure density near the maintenance threshold, the sensitivity of that threshold to price, and marginal impact. For finite cohorts, the classical Daniels-Smith fiber-bundle exponents remain the reference scaling laws. The new result is the weighted variance, including normalization fluctuations, and the resulting parameter-free prefactors for nonlinear price-mediated load sharing. Heavy-tailed exposures form a less settled regime in which rare large, conservatively levered holders can delay the loss of stability. An event-independent background-loss statistic agrees closely with the corresponding mean-field dilution prediction. Path-resolved simulations with partial deleveraging and transient impact show a finite cascade duration and stalled ignition when price impact decays too quickly.