The horse in question is a 30+ (?) yo neglected (starvation) rescue case.
Overgrown bars can lead to a variety of problems, such as exerting pressure on the hoof wall which causes it to crack.
The Right Hind hoof has a crack starting at the lateral toe quarter which began with an abscess. (As seen here).
Before trim After trim
Both bars are too long, grown above the height of the frog preventing it from being weightbearing. The lateral bar has extended almost to the frog’s apex and overlaid onto the sole. At the green arrow it is raised above the sole by at least 1/4″. The furthest point of the bar (where the blue arrow starts), exerts pressure on the wall when the foot is weightbearing, in the direction of that arrow and splits the wall apart. The underrun heels also contribute to the problem. To correct the problem, the bars have been shortened and brought back approximately to the frog’s midpoint, the heels have been lowered and brought back to the widest point of the frog, and the toe has been backed up. These changes will allow proper forces to be exerted on the hoof capsule allowing the crack to grow out.