Autism self injury is often about the loss of control. It can be a very scary and self/mind melting experience. When you lose the control, you get into a state that is angry, scared, vulnerable etc. It's quite different from a simple anxiety attack, it really hits the core of your being.

Reasons can be you did something wrong, messed up, didn't understand, was criticized etc. It might be your own doing/failure or someone else taking control from you.

The problem is really the lack of greyscale here. Either you are in control or out of control. The "cure" is to try to create a greyscale. It can be done on a kid's level too. I assume a therapist will work with this.

Basically it is learning to take a small step back in your mind instead of driving yourself onwards, learn it is not THAT bad, get the panic feeling out of the equation. Even if you cannot in this state even in the beginning of an attack, understand this in that state, you can learn to act like if you did. Act like if it is going to be OK and try to defuse the trigger. No, it is not easy for an adult either but it can be done. You need to learn to learn when you are slipping into that stage, allowing you to focus away from the failure or whatever happened, do something that you have repeated from before so this program kicks in.

The program can include something that you do with your hands, something you don't fail at, if you can stay calm enough for it. In adults it's usually more common it is a mental thing. Although I don't see why it cannot be a hands on thing in adults as well. It can be a thing like building a block/lego house or something. If it is a task causing "order" it might be extra soothing, using the built in defense mechanisms in autism.