I can see the reduced hours being legitimate though. Generic speaking, if the person has a major attitude problem, customers are probably going to pick up on it and even be the focus of it. That's going to make the shopping experience unpleasant and they will come back only when they absolutely need to, rather than want to. Get a new customer instead of a long time customer, after a round of attitude they may just not come back ever. On the coworker side, nothing like a toxic person to drive morale into the subflooring. If a certain amount of documentation is needed to fire the employee, that is just that much more time the employee has to drive away customers and make coworkers miserable. So for business self preservation dropping the hours to absolute minimum when the decision to fire has been made and only the documentation is lacking means minimal impact on customers and on coworkers and tells the problem employee that they had better start looking for a new job immediately.
I know a store manager for a different retailer, he does that when he doesn't have time to spend the weeks of verbal warning, first write up, second write up, third write up, final write up, submission to corporate HR, and he has an attitude problem person causing issues in the here and now. Same reason he hires as many people under temporary seasonal instead of permanent as he can even when he intends a permanent hire, since the documentation to fire a seasonal is far less and problem children can be let go far faster and far more easily.