Bad maintenance disabling the safety devices, or grandfathered equipment which didn't have them, or inadequate employee training on safety. All of those put Walmart at fault to varying degrees. That looks to me like the most likely scenario in the absence of other data.

Or someone intentionally jammed any safety mechanisms, which would mean that person committed murder or manslaughter depending on the details.

It's also possible that the deceased employee panicked when she realized what had happened and failed to operate a safety device she would have known full well was there if her rational brain hadn't been overwhelmed by her lizard brain. That would be tragic, but not actionable.

We still don't know enough.