That might be one of the most common of distortions.
There are many causes for flares, but here are the 3 most common.

I’ve represented them by coloured highlights and I’ll refer to them by colour to make things easier:

▪️ Green: natural conformation. This is a naturally bow legged draft horse (common for drafts) + he is varus, wears his external wall faster than the internal. On this picture, the external wall is on the left, the internal on the right.
This is how he was born, how his body developed. We’ll keep the flare from worsening but we won’t change his natural buit.

▪️ Blue: injury induced conformation. Ex International show jumper, kaput hind-end with heavy arthritis in his lumbar, sacrum, hips, stifles, hocks and fetlocks. He just copes the best he can and lives a happy retirement. The flared side is the medial side (internal). Keeping the flares in check is ok, over correcting them would be disastrous for this horse.

▪️ Pink: nutritionally weak mare with probable metabolic issues (never tested). The flares were trimmed reasonably but really got better when nutrition was balanced.