Epenthesis
A similar example is the English indefinite article a, which becomes an before a vowel. It comes from Old English ān ("one, a, an"), which kept an n in all positions. From a diachronic (historical) analysis, the original n disappears, except if a following vowel needs for it to stay: an > a. However, with a synchronic analysis people can also see it as epenthesis: a > an. This is how most native speakers see it.