Airbus doesn't close the spar valve when you set the fuel switch to CUTOFF, just the engine fuel valve (note that this is different than Boeing - Boeing closes both the engine valve and the spar valve when you select CUTOFF).
As a result, if there is a bad seal on the engine cutoff fuel valve, the pressure from the boost pumps can cause fuel to leak past the valve and into the combustion chamber. That can cause a major tailpipe fire on the subsequent start.