1) People are making a connection between “an” and “event ” in the phrase “an historical event”. They probably think that the connection between “an” and “event” is more important since “historical” is only modifying the noun.
But this is incorrect since the logic behind using “an” over “a” is to dissolve back-to-back vowel sounds (when spoken). So “an hour” makes sense not because “hour” is a noun, but because the “h” is not pronounced.

2) People assume that because we use “an hour”, they can apply it willy nilly to whatever “h” words come their way.

In either case, and from what little research I’ve done, contemporary views support the rule that “an” is only used to break up back-to-back vowel sounds. So when people use “an historical” on NPR, it’s because their snooty.

“The distinction between a and an was not solidified until the nineteenth century. Up to that time, an preceded most words beginning with a vowel, regardless of how the first syllable sounded. The U.S. Constitution, for example, reads: “The Congress shall have Power . . . [t]o establish an uniform Rule of Naturalization.” But that is no excuse for a twenty-first-century writer.
People worry about whether the correct article is a or an with historian, historic, and a few other words. Most authorities have supported a over an. The traditional rule is that if the h- is sounded, then a is the proper form. So people who aspirate their h ’s and follow that rule would say a historian and a historic. This is not a new “rule.” Even the venerated language authority H. W. Fowler, in the England of 1926, advocated a before historic, historical, and humble.”

(Under the word “an” in the USAGE section)

The remainder (history, house, hat, hill, hobo, etc.) are pronounced with the “aitch” sound and should be use with a.

Only a Cockney or an hidiot would do otherwise…

As in, “an historic event” or “a historic event”…

Perhaps the idiomatic expression has “the best of me”.