Broken joint on the third rail that tore off the third rail shoes on the train when the askew blunt end of the out-of-alignment piece of third rail struck the shoes. Relatively minor in the grand scheme of things (shoes are 'consumables' so the shop can replace the broken ones instantly), but it wouldn't have been possible to pass the point of the break without tearing off more shoes and they would've had to turn off the juice to manually inspect the damage to the shoes before a rescue train could've safely come in, attached, and backed them up. So in the end there was little they could do except turn off the power, evacuate on-site, and swallow a large delay.