They seem to have hit the row limit, not the column limit. They were using Excel to collate the data coming in from different test centres: when one of the CSVs

They seem to have hit the row limit, not the column limit. They were using Excel to collate the data coming in from different test centres: when one of the CSVs was loaded in, the row limit was exceeded and Excel cut off the extra rows without the operator realising (I'm going off reporting by Alex Hearn at The Guardian here, and he's normally pretty good)