It is well-known that the determiner a is substituted with an when the following word begins with a vowel (letter or sound). In some cases, however, an has been used preceding words beginning with (as spoken) [h].
According to Wiktionary, “like many terms that start with a non-silent h but have emphasis on their second syllable, some people precede historical with an, others with a.”
If syllabic emphasis is one explanation, how is this distinction acquired? What makes it significant enough to influence a person’s speech/writing?
My theory is that phrases such as an historian, in which the H is commonly pronounced, originated as a hypercorrection since propagated by a small number of native English speakers.