alt Hacker NewsIt's one of those things American pseudo intellectuals do because they think it's more correct and want to sound superior. Maybe they heard a professor say it that way or read it in a book. Anything but actually process why the a/an distinction exists.
Sportscasters do it too. It's all over media. You hear it in movies.
I'm pretty sure "an historic" is the norm in American media, not the other way around.