Which one is grammatically correct “reasons for cyber bullying “ or “reasons of cyberbullying “? Thank you ?

The reasons "of" would mean that you are discussing the justifications that someone might use for bullying (and even then, it would be better stated in a different way).  If you mean reasons as in causes which make a thing happen, though, it is "reasons for" the thing that happens.

'Reasons FOR cyberbullying' is the gramatically correct sentence.

"for" cyberbullying 

The correct word is for, not of.

"reasons for" ...