I’ve always heard if you hit another car from the rear,(anything behind the front bumper), then you could be at fault because you are not in control of your vehicle.
There are exceptions to that rule. For example if another car switches into your lane and eliminates your safe stopping distance, and then slams on breaks, you wouldn’t be at fault. I am in the insurance business and have won that argument many times. In the case of the accident, USAA will attempt to get their OOP expenses and will also ask for the deductible. If they want to proceed quicker, then they can file against the person at faults insurance company and won’t have the deductible issue. As a rule, your personal insurance has to deduct the deductible if you are using your own coverage. Subrogation, if successful will make you whole again, but it will take some time.