Yeah, this is a mostly American hyper correction. You see the same thing with "an herb". I'm guessing it's a relic from people who spoke French affecting the same pronunciation in English. You occasionally even see "an history"
May well be a regionalism an well. I say an herb and don't pronounce the H. But a hotel (or a history) and do pronounce the H--and hence use "a." There are a number of words that can go either way because the (acceptable) pronunciation covers both variations.
"historian" is a fairly long word. If you stop to create a word break, and then aspirate heavily on the "h", it can be uncomfortable to say. If you don't, then the "h" can get very soft if people try to say the whole word quickly and I imagine that some people drop the "h" entirely because of that, or make the "h" so soft it feels natural to contract
