I agree with the author that any fault between the zone protected by differential CTs ie Primary and Secondary of transformer will give trip command to both MV and LV breakers siomultaneously sothat transformer is isolated. The differential relay 87T will operate for the faults within the zone and not any fault occuring out side the zone. If there is a fault after the protected zone of 87T occuring say at LV bus and if the protective relay gives trip command only to MV breaker then LV breaker will not operate and bus fault continues even after ATs closes.This may be the reason of the case described here.