Weak upper back and possibly poor T-spine mobility - if you can't really extend the t-spine in the rack position, it will be more prone to collapsing under load. You may also simply not be locking it in as well as you could. Remember that even though you're protracting your shoulder blades, you need to be extending the upper back as well.
You can do good mornings and bent rows, both focusing on forceful t-spine extension. And of course more front squats - get a good volume of squats that are below the weight threshold at which you start collapsing so you're training that rather than reinforcing the collapse.