Most of the non-matched graduates, either entering the scramble or failing to match post-scramble have either one of or a combination of three traits:
1) Poor grades, professionalism, clinical skills. Just undesirable candidates at the bottom of the class.
2) Excellent grades, references, skills, etc- basically top of the class- but only apply to the most competitive residencies. These candidates are quickly snatched up in the scramble.
3) Excellent grades, but terrible social skills. Although this is unconfirmed, the unmatched graduate from several years ago from my school apparently had a 4.0. However, he was, as described to me- socially inept. I didn't get details as to what exactly this meant, but I'm assuming the individual was either riddled with social anxiety and had little people skills, was on the autism spectrum and had clinical difficulty with communication and expression, or was just an unpleasant person.
It is important to remember that the individuals deciding who is accepted into the residency are mostly the current residents- they're deciding who they want to work with for 60-70 hours a week for the next 2 years. There is a tremendous social aspect to matching that I believe is overlooked by many grads.
What happens to those who don't match? I believe the APMA created some kind of program for these individuals where, as I understand it, they basically extern for a year somewhere, and get put into the cycle process again. As long as you take care of yourself, don't worry about the matching stats. They are low enough that you should be confident you're not going to finish in the bottom 5% in your class or nationally. Even if you do, you'll probably still land a spot.