Why does Sora go anti form when he is trying to go valor form?

In kingdom hearts Sora is trying to go Valor form but it goes wrong and Sora goes into anti form. Why does it happen?

Unlike other Drive Forms, Anti Form cannot be purposefully activated. Instead, it will be randomly activated when Sora attempts to enter Valor, Wisdom, Limit or Master form based on a hidden "Anti Point" counter.
This counter is increased by 1 every time Valor, Wisdom, Master, or Limit Form is activated, but is reduced by 10 when Final Form is used, and resets to 0 when a new form is acquired. Gameplay-wise, the Anti Form exists as a consequence of over-using the Drive system, and is meant to be "strong but troublesome".

You will also never go into Anti Form, if you enter a drive form outside of combat, so if you don't have the Final Form unlocked yet (if you use mostly Final Form, you will pretty much never enter Anti Form), try to enter your preferred drive form before you enter combat.

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