This is a great article by Kathy Farrokhzad on her blog Horse Listening. See her intro below and click on her link for the whole post.
"Do you have a horse that seems to regularly trip or stumble, either in the front or hind end?The footing is good. The path is clear. There were no sudden changes to your direction.
The horse is sound and you know the tack fits well. His feet are trimmed. There are no other underlying physical issues that you are aware of.
Yet your horse stumbles here, trips there, and as time goes on, you learn to just quietly ignore it. After all, the horse is trying his best and there’s nothing you can do, right?
If you listen carefully, you might even discover that you are more a part of the equation than you give yourself credit for.
It might be something you are doing. Or it might be something you are NOT doing!
Be an active rider so you can help your horse through these moments. Your strong problem-solving skills are just the ticket to helping your horse develop better balance during riding. "