Toe + Quarter cracks

I think the main reason for cracks starting at the ground is unbalanced feet. Due to lack of or improper trimming. Cracks that start at the top are usually from abscess or puncture. This can be from small grains of sand working up the white line.

                                                                    Quarter crack
If you have a crack starting at the ground you can usually stop it by trimming or shoeing.
These cracks can also be caused by under run heels. If your horse has a curved hair line looking from medial or lateral view they will quite often break out at the hair line directly above where the heel actually makes contact with the ground. This is caused from lack of support to the buttress of heel . These will usually heal if you can get the foot to proper balance. Cracks that start at the hair line should never be clipped at bottom. This will make them worse. To fix a crack that starts at the hair line the best way I have found is to carefully stitch with a horse shoe nail. Just drive nail through the crack side ways and clench then rasp nail head down till smooth. You have to know what you are doing because you can quick them.

Some farriers will use router and take all hoof wall off in the crack. I have seen this over done causing more damage to the foot. I like to use the KISS method as much as possible.

This shows a toe crack with a stitch in it.
It is very easy to do and works very well.
Any time you have a crack at the hair line you do not want to clip the shoe because the foot will keep expanding at the top and never heal. Even just shoeing will restrict expansion at the ground so the top will not heal.
                                                           Putting a stitch in crack