Another person here that won't 'yell' at you. Accidents happen, unfortunately horses are extremely unpredictable creatures and also seem to be very accident prone. You can always do things to reduce the chances of an accident, but often you can't entirely prevent it.
With an ottb as well, they do have a tendency to get a bee in their bonnet, and once they panic, you can't do a hell of a lot other than stand in the middle or sit quietly in the saddle and wait for their brain to come back down from space.
As SJ said above, if you have a roundyard available, I would strongly suggest that you use it for a while to get some control of him on the lunge before lunging in the open. Were you lunging in a bridle or halter? If a halter, try putting him in a bridle and either run your lunge line through the inside bit ring, over his poll and clip onto the outside bit ring /OR/ Run the lunge line through the inside bit ring and clip to the roller/girth points.
Both of these methods of attaching a lunge rein will give you more control. If a horse goes to bolt on the lunge and you can pull its head around to the inside, you will have a very good chance of stopping the bolt. The second method, with the lunge rein connected to the girth or roller, I have found to be the most effective on horses that act up on the lunge. You can't hit them in the mouth, only bring their nose to the inside. When they're more advanced I will attached an outside side rein, and keep the running inside rein so that you still have a good degree of control if you need it, but then you start working on an outside rein contact as well. Two birds with one stone ;)
I hope Mustard heals quickly so you can get back to letting him know that that behaviour is not on!!
Kayty, I am trying to visualize this. The girth, do you mean between his forelegs? Or do you mean on the side of the girth, to the inside of the lunging circle, then unclip and change it to the other side when changing directions? I guess I could clip it to the buckles of the girth . Or the D ring of the saddle (where the bucking strap would clip on) but that is pretty high. Do you want somethign that is mid height on the hrose,?