Is there a trick to getting things straightened out after the horse kicks up and one leg comes down outside the trace? This happened to me twice in the last two weeks, the first time I was able to back her to the singletree, this lowered the traces and then she stepped forward on the inside where she belongs. But today that didn't work and I finally had to stop and unhook the trace chain. She got a little rope burn on the inside of her leg before we were done. Does this mean I have the traces hooked too long? too short? I didn't want to stop and rehook as soon as it happened for fear of teaching her that the reward for kicking is an immediate pause in the work at hand.

I just keep driving them when it gets real sore they will step back over.

Based on your description yes to loose or to long. However, there are many reasons horses step out of traces.

Let her work that way a while and see if she works it back over. A little burn might be good for her. If she's kicking at it it doesn't matter how long or short it is she'll get her leg over. I'd be careful working around her legs and the tugs.

Do you use a crupper on that harness? I mention it because a too narrow crupper on a horse with a wide tailhead could inspire her to kick. What you've said about this horse makes it sound like the kicking is related to some discomfort.

No crupper
I thought about some discomfort as well. I posted the question about how tight the girth should be. She has had scratches all summer, but I am treating them every day and there are no raw areas any more, just small crusts you can feel around her pastern. She seems to get more upset walking in tall rank grass and weeds, maybe it's the scratches. It usually happens after we have been out for 20-30 mins. The tugs are 3/4 inch twisted nylon, very soft.
She used to throw her head but I trimmed the guard hairs around her eyes and switched to a mullen bit.
I am concerned about her kicking back while I hook or unhook the traces. I lay out both traces on the ground before I hook either one up and I try to work from the side, still if she were to kick back at that moment I would stop posting.

I had one this spring that was really touchie with her back feet . It took me all summer long to get past that, a lot of time and more work than I was prepared for. She is doing great now ,but be for I was able to handle her feet safely ,I would keep working her slowly and try to give her a chance to get back in on her own. She stop doing it.
Shanen

Scratches can be very irritating and if she just starts kicking out while walking along, for no apparent reason, it could be that. The best success I've had of treating scratches is to shave the hair off so air can get to it. We definitely don't want you to get hurt.

Not a good idea to lay the traces/tugs down. Have them hooked to the harness spider and then you can reach up to get them. If they are down, you have to bend over and get them and your head is too good a target!

i like to sack out new horses in a round pen, make sure I can work around there legs in the field.
But generally green horses have caused some issues with unhooking their trace chains and stepping outside of them.
So i may put one round of duct tape to keep the trace chains from unhooking from a fidgeting horse.
If a green horse steps outside the the traces, I do what KC and Mooney said.
Hard work, lots of rest, they learn to appreciate the rest and stand still during the stop.
I don't worry about a burn on their leg. An oldtimer told me, ' a little scrathch on the leg is a long way from the heart. It won't kill them, and most will learn pretty quick.."

Had a mare kick and get over the tongue. We went about 1/2 mile sorta fast before we hit a drain ditch. When the cart dropped down in it, she was able to get her leg back over the tongue and then the other horse would finally stop. She never did anything like that again. Lessons learned on the horses own are never forgotten. Specially ones like opening doors or others you don't like.