Maintenance sorties are by their very nature, hazardous non-standard operations. My sincerest condolences to the families involved and to Air New Zealand staff on this tragic (and infamous) day.
I hope the cause is quickly established, procedures modified and lessons learnt to make this type of operation safer for all of us whom partake in them.
Due to the many complex technical procedures involved with most maintenance/check flights, especially with an aircraft as advanced as the A320, I would say speculation is absolutely pointless.