Note that "they" in this case is the new parent company. IIRC, they were a fairly new bank, heavily reliant on technology. They had the tech but not the customers. TSB were the opposite. The parent thought their tech, which up to that point was only dealing with 100,000s of records, could deal with billions with little change. They were spectacularly wrong and it showed in all testing. But their management pushed ahead to go live anyway.

Their actions are not far from criminal negligence IMHO.