Yes that would be why it made that illusion. He was actually going in a very large left circle. I'm glad you found it helpful but I hope you don't have to deal with a harness like this!Ahhhh, that explains why it looked as if he was pushing his left shoulder forward and bending to the right.
I couldn't figure out the slack either, and the left trace seemed loose. I'm glad people post pictures like this, then explain the solution. It helps me to catch things with my own horses, as well as what I might be seeing when out with others.
I've had the bridle come apart once or twice while I am driving and I always have to check if anything has come unbuckled while it's in its bag. I would never ever use it with a green horse. I used to have an Amish made leather harness that was supposed to be A size but was gargantuan! It did not fit a 36" horse, much less a 34" so I ended up selling it and this is the only one I have now. But when I still had it, I would use the nylon backsaddle and reins with the leather bridle and breastcollar with traces. Come to think of it, I should buy a true A size leather bridle and breastcollar w/traces!