What happens next?

How to prevent litter box aggression
Add another litter box.
The main cause for litter box aggression in cats is too few litter boxes in the home. Simply adding another litter box can often dispel much of the tension that is causing your cat to act aggressively.
The general rule of thumb is that you should have one litter box per cat, plus one. For example, if you have two cats, you should try to have three litter boxes in your home.