How would things be out of stock on the day of delivery

I’ve been ordering on takealot for years and never had a problem until recently… I ordered a bulk of products , and we’re scheduled for delivery only for them to cancel the order on the day of the delivery telling me that the products are out of stock… how?? When I paid for them they were not out of stock .. I honestly don’t understand how that can happen.. things will be out of stock on the day of the delivery..