You have one answer from G. That is on target.
Forging comes in 2 varieties. In both cases the back foot hits the front foot(or bulb) One, the back foot hits the bottom of the front hoof. Two, the back foot hits the top of the front shoe and/or the bulb.
Forging is undesirable in either case. There are 3 major causes for forging.
One, is conformational. Two, trimmed incorrectly. And three, improper rider.
Let's dispel some myths.
Gaited horses, like 3 gaited horses, move better at, or near(plus or minus about 2 degrees) their natural angle. The natural angle for most horses fall in the range of 50 to 55 degrees for the front, and 52 to 58 degrees in the rear. It is a rare horse that has a natural angle below 50 degrees.
Forging almost always occurs when going slow. And most often when they are either being lazy, or the rider has fallen asleep and forgets to ride the horse.
Now to the horse in question. Yes, the horse is a downhill horse, ie, the back is higher than the front. Not a good thing, but not terrible either. These horses have a much more difficult time staying balanced and require a lot more rider interaction to hold gait, stay balance, etc.
If the current hoof angles are as the photo shows, it would appear she is too low in the front and rear. And the toes are far too short. Horses that have the running walk as their preferred gait, usually need a front foot that is heavier than the back foot.
Another common cause of forging is the bit. If the bit is not right, the horse will pay more attention to it and less attention to where it is going and how it is going. This is usually worse when moving slow, because at slow gaits(usually the walk) it has more time to think about that horrible thing in its mouth. If you are using what they commonly call a walking horse bit, ie, long shanks, high port, throw that dang thing as far away as you can. The horse does not exist that likes this bit, yes, they get used to it because they are forced to do so. First item to check in finding the correct bit, is the floor of the horse's mouth, ie, how much room is there for their tongue when there's a bit in the mouth. You may find you need a bit with a comfort rise that gives more room for the horse's tongue. And you do not need high ports and long shanks, period. They are training short cuts that should be avoided on all horses.