About a week after BA's system outage first explanations appear in the media:
https://www.thetimes.co.uk/article/ba-power-fiasco-blamed-on-staff-blunder-tbfhxwsw2
But this is not undisputed:
https://www.theguardian.com/business/2017/jun/02/ba-shutdown-caused-by-contractor-who-switched-off-power-reports-claim
Reminds me of an incident I had witnessed (not caused) as a student at a data centre of a bank about 1990, where an operator, having just returned from holiday, started his usual batch jobs with a punchcard... Unfortunately the processes were changed when he was on holiday.
The data centre was out of operations for two days... Only person happy about this was me - full pay for no work for these days ;-)