If they are saying the door was opened by ground staff. Was that from the outside? If so, and the chute deployed then there is either a fault with the door mechanism or someone is telling porkies.
If it was opened from the inside then it is the responsibility of the individual opening the door to check it is disarmed.
That individual has to be appropriately trained and authorised to do so. If not then they shouldn't have been opening the door.
Some airlines do not allow ground staff to open doors (from the inside) if cabin crew are on board. An attempt to avoid the scenario you have described.