My theorie is that since Sora became a heartless once, it means he has darkness in his heart and the forms are "fusions of hearts" since you lose party characters.
Then when you morph in a form, something would go wrong and the darkness from Sora's hearts will overcome him transfroming him a heartless...
Anti-Form is a form that transform sora in a heartless.He is very fast in this form and has a lot of combos ,but he takes 50% damage,don't have keyblade,HP orbs don't cure and Drive orbs just uses more of the drive gauge.
When you enter this form, you will always have 5 bars of form and when all the bars are gone you return to normal with 0 Drive bars.
There are chances of transforming into it based on anti-points.The more you have, bigger are the chances of transforming into it.
Every time you use Valor,Wisdom,Limit or Master Form you gain 1 anti-point.
Everytime you gain a new form you lose all anti-points.
Everytime you transform into anti-form you lose 4 anti-points.
Everytime you transform into Final form you lose all points.
In special battles the chances are multiplied from 2 to 10 times depending on the battle.
PS:You can't transform into anti-form while you are without enemies on the screen.If you enter in a safe place with anti-form, the form will revert.
PS2:In KH2FM, the ability from the keyblade you gain from roxas, makes the chances of going into Anti 100%,but you can revert it.