Decaphobia (from deka, Greek for "ten") is the fear of number 10. One of the reasons for the fear could be that the Roman numeral for ten is X, which is linked to the fear of letter X. So they think that X should be the Roman numeral for 0 or that X should not be the Roman numeral for 10. The fear can be caused by negative plea feelings of other having scores of 10/10 in games (especially when the self couldn't achieve it) as well as in ratings that they obsess on getting it only not liking many of them.
Decaphobia is more suffered by the Kikuyu community in Kenya, members of the community fear the number 10 so much to the point they never utter the number even when counting, members of the community would say "full nine" instead of "ten" to avoid bad luck. Decaphobes fear that counting to ten spells doom for them.