I agree with you that anti-form is the most annoying form you'll ever get.
But we won't get it by random.The rate of changing anti-form depends on the dark points that isn't visible to us.
Anywho,Yes I think it has to do something with him becoming a heartless.
Anti - Form is only a heartless version of Sora.
Maybe because of him have such a strong heart,his heartless is stronger than any shadow.
Spoiler:
Well,we all know that he became a heartless in KH 1 and nobodies are only created with those who have strong hearts.Since Sora has a strong heart,that what made Roxas.
Sora changing into a heartless again maybe an idea for Square-Enix.So,maybe this is one way to do that again.So it will connect with the event of Sora became a heartless.After playing KH 2 many times,I felt there was something to do with the event.
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But I have another opinion.Maybe it was just a way that Square-Enix gave us to have more fun and challenge.For letting it being just a normal form like others but with it's own side effects.