Walmart cancelled grocery delivery order?

I was scheduled to receive a medium size delivery from Walmart today. It was for a grocery order I placed over a week ago. But at some point this morning, they cancelled my order.

I wasn't informed about the cancellation and no reason is listed. I only found out that it was cancelled when I checked online after the order was late. I spent over an hour on the phone trying to get in touch with someone, but their line kept dropping me every 15 minutes.

Does anyone have any idea why Walmart would cancel an order? Is this common now? If I reschedule the order and wait again, is it likely that they will cancel my order again at the last minute?