This is because latin nouns ending in "-<vowel>-x" are of the third declension (consonantic) and the nominativus pluralis is "-(i)ces". For instance: "vortex"-> "vortices", "codex" -> "codices", etc..

At least this is the correct plural, whereas one can often see "forae" as a plural of "forum". This wrong because neutral words in the second (o-) declension have the ending "-a" in the nominativus pluralis, therefore "fora".

bakunin