It's not really a question of what started the problem - as problems happen. BUT how it rippled throughout their network and, then, why it took them so long to get it all back. The usual barriers that prevent corrupted data flowing outward appear to have failed and the sequence in which the systems were restored, appears to have been deficient.
Every global corporation must be (silently) thanking BA and warning them of what can happen and givem the chance to check their plans.