It has an accent because there is a hiatus formed by a weak vowel (i, u) together with a strong vowel (a, e, o). This fact is marked with an accent on the weak vowel. Although it is a flat (or grave) word ending in 'n', 's' or vowel and would not have an accent according to the general rules, the formation of hiatuses by the accent of the weak vowel takes precedence over any rule.
